SALT LAKE CITY -- Rookies Jayson Tatum and Markelle Fultz already were familiar foes before reaching the NBA, and they will be forever intertwined because of the blockbuster trade that delivered them to their respective Atlantic Division homes.

How appropriate then that, pitted against each other in their NBA debuts, the two found themselves dueling in the final seconds of the Boston Celtics' 89-88 triumph over the Philadelphia 76ers Monday in the opener of the Utah Jazz Summer League on the University of Utah campus.

Tatum hit a go-ahead, pull-up jumper with 5.7 seconds remaining to cap a 21-point performance, while Fultz was blocked by Boston's Jaylen Brown trying to muscle home a reverse layup at the opposite end as the Celtics escaped with the win.

"Me and Markelle have known each other for a long time, from high school we played against each other, all the All-Star games, McDonald's [All American Game], Hoops Summit, Jordan Brand," Tatum said. "[Now] we finally reached our dream and I'm pretty sure we're going to compete against each other for a long time."

The Celtics, having swapped spots this year with the Brooklyn Nets as part of the Paul Pierce/Kevin Garnett blockbuster, landed the No. 1 selection in May's draft lottery. Boston traded that pick to Philadelphia in June, bringing back the No. 3 selection and a future first-round pick, which will be the Los Angeles Lakers' 2018 first-round pick if it lands between spots 2-5, or the Sacramento Kings' 2019 first-round pick if it's not the top overall selection (otherwise Boston receives Philly's 2019 first-rounder).

Celtics coach Brad Stevens, knowing the growing hype surrounding the Fultz-Tatum showdown to kick off Boston's summer slate, texted his rookie before Monday's game and advised him to relax.

The 19-year-old Tatum certainly didn't show signs of being under pressure in the closing seconds. With Boston operating without a timeout to draw up a play, Tatum calmly got to the left elbow and pulled up over a strong contest. The way he worked in isolation and then faded away slightly bore obvious similarities to Celtics legend Pierce, to whom Tatum has compared his game.

"I was really just trying to give [Tatum] the ball and get everyone out of his way," Celtics summer league coach Jerome Allen said. "Just like I drew it up, and I didn't draw up anything. It was a big shot. Not easy."

Fultz had a chance to respond. Catching an inbounds pass on the right wing and with space to drive the baseline, he fought through Abdel Nader's contact to get to the opposite side of the rim. Second-year forward Brown shuffled over with help and denied the shot. It was Tatum who swooped in to bat the ball away and prevent a second-chance opportunity.

Tatum's 21 points came on 8-of-17 shooting. He missed four of the five 3-pointers he took but had a couple of loud finishes, including a thunderous one-handed jam late in the first half that left the crowd oohing after he ripped through a defender to get to the rim. Tatum added seven rebounds, five steals, three assists and one turnover in 33 minutes.

Fultz played only 22 minutes and finished with 17 points on 6-of-16 shooting. He had three blocks but also four turnovers.

Still, Fultz dazzled with his offensive moves. Enough so that Golden State Warriors forward Kevin Durant, a fellow DMV native, took to Twitter to praise Fultz's moves.

Fultz got some 💩 with him. The hesi pull-up jimbo was smooth. Probably don't understand what I just said if u don't REALLY hoop — Kevin Durant (@KDTrey5) July 4, 2017

Fultz also offered high praise for Tatum. The two will see each other again at the Las Vegas Summer League, likely twice in the preseason, then four more times during the 2017-18 regular season.

Tatum's heroics helped him steal the spotlight Monday, but it was Brown who had maybe the most impressive outing. The Celtics had planned to pitch Brown on taking it easy this summer after a long rookie season that culminated with an elevated role in the Eastern Conference finals.

Instead, Brown stormed into Danny Ainge's office after the season and demanded to play in summer league. Brown wanted to use the summer to display the improvements in his game and show the rest of the league that he's ready to take a step forward this season.

Brown scored a game-high 29 points on 10-of-16 shooting. That's the most points he has scored in a game since high school -- which wasn't all that long ago for the second-year player who was drafted after one season at Cal.

Asked if he expects his minutes to be limited, Brown said he's at summer league to compete.

"No offense to Utah, but I don't want to do nothing else out here but play," Brown said. "I came here to play basketball."

Brown likes what he saw from Tatum, a player he has taken under his wing already in recent weeks. They even have a nickname, 7/11, after their jersey numbers.

"I just told Jayson to just take a deep breath and breathe," Brown said. "Last year when I was in his spot, I was so nervous. I think my first game last year I shot 17 free throws and I only had like 15 points. I got to the free throw line a lot, but I was so nervous.

"I could see it in his eyes he was nervous early. I just told him to take a deep breath, let the game come to you. He made some tremendous plays. The boy can ball. Can't nobody take that from him."