Floyd Mayweather, who retired as pound-for-pound king in September, was notoriously hard on sparring partners, often taking them on like it was a real fight and grinding through them during rigorous training camps.

When Mayweather was preparing to fight Robert Guerrero in 2013, a young fighter was brought in to give him left-handed sparring. But this fighter didn't just give Mayweather good work. He impressed Mayweather, which is hard to do.

The young man? Errol Spence Jr., a 2012 U.S. Olympian, a dynamic fighter with the complete package of skills, speed, power and work ethic, and now the 2015 ESPN.com prospect of the year.

Rafael's Prospect of the Year 2015: Errol Spence Jr.

2014: Felix Verdejo

2013: Vasyl Lomachenko

2012: David Price

2011: Gary Russell Jr.

2010: Canelo Alvarez

2009: Daniel Jacobs

2008: Victor Ortiz

2007: Amir Khan

2006: Andre Berto

2005: Joel Julio

2004: Samuel Peter

2003: Jermain Taylor

2002: Miguel Cotto

2001: Francisco Bojado

2000: Julio Diaz

"When I was getting ready for Guerrero, the first guy I started looking to spar with was a southpaw and Errol was giving me real good work," Mayweather said. "He pushed me and made me get in tip-top condition, and once I was in tip-top condition, I was ready and the best I could be. He's a hell of a fighter."

Mayweather went on to brag about Spence's talent and said he deserved an immediate shot at titleholder Keith Thurman.

Those were huge compliments to Spence (19-0, 16 KOs), 25, of DeSoto, Texas.

"It's an honor for him to stand behind me and say I'm the future of boxing, and I'm not even signed with [Mayweather Promotions]," Spence said. "He obviously believes in me. I have to prove him right by winning titles and beating these top fighters."

Spence began boxing at 15 when his father, a Lennox Lewis fan, took him to a Dallas gym.

"It really wasn't my choice," Spence said. "I came home from school and 20 minutes later we pulled up to a boxing gym. He didn't tell me where we were going. What's going on? We going to pick up something? And then we're in a boxing gym and he's asking the coach when can I start, and I started the next day."

Now, as Spence moves closer to a major fight, he feels training with Mayweather was big for his development.

"The intensity level of sparring felt like a real fight," Spence said. "We were sparring 5-6 minute rounds with 30 seconds rest. I handled myself great. I only had a few pro fights then but I got a lot of experience. It was a big deal. I watched Floyd every second I could. I really just wanted to beat him up. He's the best in the world so I wanted to prove a point and make a statement. I think I definitely opened some eyes."

Spence is said to have given Mayweather a black eye in one session, not to mention widespread word that he later knocked out Adrien Broner in sparring. Spence, however, is not one to punch and tell. When asked about the stories, he chuckled, adding, "I don't know anything about all that."

Regardless, most view Spence as a lock to win a world title and expect that he'll eventually find a place on the pound-for-pound list. He had a tremendous amateur career -- three U.S. national championships (2009-11) and two National Golden Gloves titles (2009-10) -- and was tabbed my most as the best pro prospect from the 2012 U.S. Olympic team. He has done nothing but enhance that view.

Managed by Al Haymon and trained by Derrick James, Spence was 4-0 (all knockouts), in 2015 against decent opposition (Samuel Vargas, Phil Lo Greco, Chris van Heerden, Alejandro Barrera) with a combined record of 97-5-2. He wants a significant fight in 2016.

"A lot of people have high hopes for my career," Spence said. "I'll stay in the gym and prove everybody, and myself, right, that I'll be a top pound-for-pound fighter, win titles in multiple weight classes, and beat the best fighters out there."

Here's the rest of the top 20 rising stars (with age, home, division, record):

2. Anthony Joshua (26, England, heavyweight, 15-0, 15 KOs)

Anthony Joshua defeated Dillian Whyte with a seventh-round knockout in December. Richard Heathcote/Getty Images

The 6-foot-6, 245-pound Joshua has great size and power and is built like a truck. He won the 2012 Olympic super heavyweight gold medal at home in London and looks like he has a legitimate chance to be the next heavyweight king. There are huge expectations for him but he has handled the pressure with ease. In 2015, he won all five of his fights, including a serious test Dec. 12 versus amateur nemesis Dillian Whyte, who took him past the third round for the first time and gave him some trouble before ultimately succumbing to his power in the seventh round.

3. Felix Verdejo (22, Puerto Rico, lightweight, 19-0, 14 KOs)

Felix Verdejo Alex Menendez/Getty Images

Puerto Rico's next big star was a 2012 Olympian and 2014 prospect of the year, and remains one of boxing's most talented and exciting up-and-comers, not to mention being a big draw at home and in New York. In 2015, Verdejo, a close friend of island legend Felix Trinidad, made his HBO debut in a dominant win versus then-unbeaten Ivan Najera, but was limited to three fights (two KOs) because of a six-month layoff and surgery to remove bone spurs from his left hand.

4. Oleksandr Usyk (28, Ukraine, cruiserweight, 9-0, 9 KOs)

AP Photo/Efrem Lukatsky

Southpaw Usyk won the 2012 Olympic heavyweight gold medal and 2011 world amateur gold during a storied amateur career in which he won around 400 fights. He has a ferocious, crowd-pleasing style, is a showman and draws crowds. Backed by the Klitschko brothers' K2 Promotions East, Usyk won his three 2015 bouts by knockout versus solid opposition sporting a combined record of 52-5-2. He's in a mandatory position, so expect to see him in a world title fight in 2016.

5. Egidijus Kavaliauskas (27, Lithuania, welterweight, 11-0, 10 KOs)

Insiders call him a beast. He's an aggressive, puncher trainer Robert Garcia describes as a Gennady Golovkin/Ruslan Provodnikov combination. He was a 2008 and 2012 Olympian with around 400 amateur fights who is in the same Egis Klimas-managed stable that boasts light heavyweight titlist Sergey Kovalev and featherweight titlist Vasyl Lomachenko. A ruptured right biceps cost Kavaliauskas most of 2015, limiting him to two late-year fights. If healthy, he's a good bet to be his nation's first titleholder.

6. Joseph Parker (23, New Zealand, heavyweight, 17-0, 15 KOs)

Joseph Parker Duco Promotions Limited

Parker, 6-foot-4, 235-pounds, who has sparred with ex-champion Wladimir Klitschko, is a fresh face in a division needing new blood. He's a big puncher with good skills and excellent conditioning. Already an attraction at home, Parker (who has had two U.S. fights) has a good team behind him, including noted trainer Kevin Barry. In 2015, Parker went 5-0 (all KOs), including versus experienced foes Bowie Tupou and countryman (and former title challenger) Kali Meehan in a huge all-New Zealand fight.

7. Erickson Lubin (20, Orlando, Florida, junior middleweight, 13-0, 10 KOs)

Erickson Lubin AP Photo/Wilfredo Lee

Lubin, a lanky southpaw known as "The Hammer," went 143-7 as an amateur and was considered a slam-dunk for the 2016 U.S. Olympic. But he instead signed a pro contract on his 18th birthday and has so far shown the total package against a solid batch of opponents, including Michael Finney and Orlando Lora in 2015, a year in which he went 5-0 (4 KOs). He's young and patience is needed but the sky's the limit for this kid.

8. Sergiy Derevyanchenko (30, Ukraine/Brooklyn, New York, middleweight, 7-0, 5 KOs)

Sergiy Derevyanchenko Rosie Cohe/Showtime

"The Technician" was a 2008 Olympian and finished a brilliant amateur career 390-20. Then he went 23-1 in the World Series of Boxing, so he's advanced despite so few pro bouts. He can box or bang and is an intelligent fighter who is probably ready right now to fight top opponents. In 2015, he was 4-0 (3 KOs) and blew away experienced Jessie Nicklow and outpointed former title challenger Elvin Ayala.

9. Oleksandr Gvozdyk (28, Ukraine, light heavyweight, 8-0, 6 KOs)

John Leyba/The Denver Post/Getty Images

Gvozdyk, who trains with Robert Garcia, was a 2012 Olympic bronze medalist and is part of the Egis Klimas-managed stable that boasts light heavyweight titlist Sergey Kovalev and featherweight titlist Vasyl Lomachenko. Gvozdyk is a well-schooled, mature technician with a good jab who's patient and wears opponents down with a precise attack. In 2015, he went 4-0 (3 KOs) and beat experienced opposition, including Cleiton Conceicao, Francisco Sierra and Cory Cummings.

10. Oscar Valdez (25, Mexico, featherweight, 18-0, 16 KOs)

Oscar Valdez AP Photo/Mark J. Terrill

The hard-working Valdez is an exciting fighter on the brink of a major fight. He was a two-time Mexican Olympian and the only Mexican to medal at the amateur world championships, claiming bronze in 2009. He has good power and boxing skills, but seems most comfortable brawling. In 2015, Valdez went 4-0 (3 KOs) and made his HBO debut. In September, he looked superb stopping ex-junior featherweight title challenger Chris Avalos in the fifth round for his most notable victory.

11. Ievgen Khytrov (27, Ukraine/Brooklyn, New York, middleweight, 12-0, 11 KOs)

Khytrov had a huge amateur career (reportedly 480-20), including a gold medal at the 2011 world championships and 2012 Olympic berth. Despite not turning pro until late 2013, Khytrov is moving quickly and promoter Lou DiBella has compared him to a younger Gennady Golovkin. In 2015, Khytrov was 5-0 (4 KOs) while raising the level of his opposition with wins versus experienced foes Jorge Melendez and Nick Brinson.

12. Marcus Browne (25, Staten Island, New York, light heavyweight, 17-0, 13 KOs)

Marcus Browne Al Bello/Getty Images

The 2012 U.S. Olympian, a southpaw, was a decorated amateur, winning numerous national tournaments and three New York Golden Gloves championships. As a pro, the likable Browne has good skills, size and a fan-friendly personality. He continued to improve in 2015, going 4-0 (3 KOs) against a solid group of experienced opponents, Francisco Sierra, former titleholder Gabriel Campillo, Cornelius White and Aaron Pryor Jr., and looked sharp against all of them.

13. Callum Smith (25, England, super middleweight, 18-0, 13 KOs)

Callum Smith will compete in the World Boxing Super Series, choosing the eight-man tournament over a fight for a vacant world title. Dave Thompson/Getty Images

A pro since 2012, Smith has moved quickly and could be the best of the fighting Smith brothers, Paul (former super middleweight contender), Stephen (junior lightweight contender) and Liam (junior middleweight world titlist). Callum, who has excellent size at 6-foot-3, went 3-0 (2 KOs) in 2015 and won the British title with a supremely impressive first-round destruction of Rocky Fielding in November.

14. Jose Ramirez (23, Avenal, California, junior welterweight, 16-0, 12 KOs)

Jose Ramirez Chris Farina/Top Rank

Ramirez was a superb amateur, going 145-11, winning 11 national titles and making the 2012 U.S. Olympic team. He has shown speed and power and is a good body puncher, but his most dangerous weapon is a fight-altering left hook. He was 3-0 (2 KOs) in 2015 because a thumb injury cost him a September date. Besides his talent, he's a bona-fide box office attraction, drawing a 13,120 for a December fight in his home region of Fresno, California.

15. Takuma Inoue (20, Japan, junior bantamweight, 5-0, 1 KO)

Inoue is the gifted younger brother of junior bantamweight titlist Naoya Inoue (8-0, 7 KOs), already a two-division titleholder at 22, and could be just as good. Takuma, 52-5 as an amateur and a former Japanese high school national champion, doesn't have the power of his older brother but is a similar prodigy who has faced quality opposition and probably will challenge for a world title in 2016.

16. Joseph Diaz (23, South El Monte, Calif., featherweight, 19-0, 11 KOs)

Joseph Diaz Jr. Ed Mulholland/Golden Boy/Getty Images

A 2012 U.S. Olympian, Diaz fights with a joyful exuberance and has shown tremendous improvement since turning pro. Diaz is not a huge puncher, but he can box and also likes to mix it up and please the fans. In 2015, Golden Boy kept him busy and he won all five of his bouts against credible opposition. He's likely to get a shot to fight on HBO in 2016.

17. Jesse Hart (26, Philadelphia, super middleweight, 19-0, 16 KOs)

Jesse Hart Al Bello/Getty Images

Hart, 6-foot-2 with speed and long arms, has boxing in his genes as son of 1970s middleweight contender Eugene "Cyclone" Hart. He was a standout amateur (85-11) and won the 2011 National Golden Gloves and USA Nationals, and just missed a 2012 Olympic berth via double-tiebreaker. In 2015, he went 3-0 (all KOs) and is so highly thought of by Top Rank boss Bob Arum that he was given a coveted spot on the Floyd Mayweather-Manny Pacquiao undercard.

18. Antoine Douglas (23, Burke, Virginia, middleweight, 19-0-1, 13 KOs)

Antoine Douglas looks to bounce back from his first loss with a bout against Ernesto Berrospe. Rosie Cohe/Showtime

Born prematurely to a drug-addicted mother, shuffled from one abusive home to another and often going hungry, Douglas has been a fighter all his life. Now reunited with his mother, an animated ringside figure, Douglas has a chance to make it big. He's athletic with a pleasing style, solid skills and a deep desire to succeed. As a "ShoBox" regular, Douglas won his three 2015 fights by knockout against two unbeaten foes and experienced Les Sherrington, whom he dropped five times.

19. Robert Easter (24, Toledo, Ohio, lightweight, 16-0, 13 KOs)

Robert Easter Harry How/Getty Images

The first thing that jumps out at you with Easter is that he is 5-foot-11 with a 76-inch reach, both tremendous for a 135-pounder. Easter, with good power in both hands, had a strong amateur background (2012 U.S. Olympic alternate) and has sparred with many quality pros, including pal Adrien Broner, brothers Lamont and Anthony Peterson and Hank Lundy. Went 4-0 (4 KOs) in 2015, including an impressive third-round demolition of experienced Juan Ramon Solis in October.

20. Regis Prograis (26, Houston, junior welterweight, 16-0 13 KOs)

Regis Prograis Rosie Cohe/Showtime

A southpaw, Prograis went 87-7 as an amateur (including in the 2011 Olympic trials) and took a big step forward in 2015, going 4-0 (2 KOs) and defeating then-unbeaten Amos Cowart and Abel Ramos on "ShoBox." Prograis, who is from New Orleans but was displaced to Houston by Hurricane Katrina, is aggressive, busy (often throwing 100-plus punches per round) and has a crunching body attack. He has gained valuable experience sparring with the brothers Jermell and Jermall Charlo.