A promising, young prospect picked up the biggest win of his career against a "name" opponent fighting past his prime. It's a common story in combat sports.

On Sunday, it was Mexican featherweight Yair Rodriguez battering a 38-year-old BJ Penn, who came out of an 18-month retirement.

Rodriguez dropped Penn in the second round with a front head kick and right hand. He followed that combination with a long series of punches and hammerfists on the ground, as referee John McCarthy waited what felt like an eternity to stop the bout. The official stoppage came at the 34-second mark.

The 145-pound headlined UFC Fight Night at Talking Stick Arena in Phoenix. It was Penn's first appearance since he retired in July 2014, following a one-sided TKO loss to Frankie Edgar.

"This was an amazing fight for me," said Rodriguez, who improved to 6-0 in the UFC. "I fought a legend tonight. He deserves more respect than I can give him. Thank you for accepting this fight."

Featherweight Yair Rodriguez used his speed and striking to break down BJ Penn and spoil the UFC Hall of Famer's comeback from retirement. Jeff Bottari/Zuffa LLC/Zuffa LLC via Getty Images

Penn (16-12-2) did not speak in the Octagon but walked out on his own. The defeat was the fourth knockout loss of his career and drops his record to 1-6-1 in his past eight contests.

Although the finish came in the second round, the fight was essentially over in the first.

Penn, who is from Hilo, Hawaii, did well enough closing the distance on Rodriguez initially and looked to work for takedowns along the fence.

Rodriguez (11-1) was too physical and his takedown defense was too good, however. He worked off the cage when Penn managed to get him there and landed hard kicks to the legs and body at range. He rattled Penn with a left head kick midway through the first round, which basically stopped Penn's aggression for the rest of the fight.

Once Penn wasn't actively working to get to the inside, he was a sitting duck for the 24-year-old Rodriguez, a promising fighter for the UFC based on his skill and appeal to the Mexican market. He is originally from Chihuahua, Mexico, but now fights out of Chicago.

Later in the first round, Rodriguez grazed Penn with a spinning wheel kick. He threw another hard left head kick that Penn appeared to block, but was still rocked by.

Penn, who trained with Greg Jackson and Mike Winkeljohn, in addition to longtime coach Jason Parillo, is widely considered the greatest lightweight in mixed martial arts history. He is a former UFC champion at 155 and 170 pounds and remains one of only three men to ever win titles in multiple weight classes.

Lauzon takes split decision from Held

In a surprising twist, Joe Lauzon, right, questioned the judges' scores following his victory over Marcin Held. Jeff Bottari/Zuffa LLC/Zuffa LLC via Getty Images

Lightweight Joe Lauzon (27-12) picked up his 14th win in the UFC, defeating Marcin Held (22-6) via split decision (29-28, 29-28 and 27-30).

The split scorecards led to a rare moment, as Lauzon admitted he "100 percent" did not agree with the judges and thought Held won the fight. ESPN.com scored the fight 29-28 in favor of Held.

A former title challenger in Bellator MMA, Held leaned heavily on his wrestling to control Lauzon on the floor. The strategy put him in harm's way early, when Lauzon landed a series of hard elbows to Held's head as he worked for a takedown.

Held appeared to make up for that sequence as the fight progressed, however. He had success taking Lauzon down, although he did not score a ton of offense from top position. Lauzon attempted an armbar in the second round, but Held calmly worked through it.

The third frame was slow, with both fighters tiring. Held scored some knees to the body in the clinch, and he was credited with two more takedowns.

Since signing with the UFC, Held, from Poland, has gone 0-2, with losses to Diego Sanchez and Lauzon, who is from Brockton, Massachusetts, and trains out of his own gym in his home state.