Teddy Atlas breaks down what Gennady Golovkin did well as the larger fighter and how he should have come away victorious. (1:36)

LAS VEGAS -- When Canelo Alvarez was awarded a surprising draw against Gennady Golovkin on this Mexican Independence Day weekend last year, there was heavy-duty controversy.

But this time, in another hard-fought battle, Alvarez claimed a majority decision victory in the year's biggest fight to take the unified middleweight world championship from Golovkin on Saturday night before a sellout crowd of 21,965 at T-Mobile Arena.

Alvarez's victory ended Golovkin's undefeated run and his historic 160-pound title reign at a division-record 20 defenses, a streak he shares with Golden Boy Promotions partner Bernard Hopkins. Hopkins was ringside and had made no secret in the lead-up to the fight that he wanted his man to win not only for the sake of the company but for his record.

Judges Dave Moretti and Steve Weisfeld scored the fight 115-113 for Alvarez, and Glenn Feldman scored it 114-114. ESPN.com also scored it 114-114.

"I showed my victory with facts. He was the one who was backing up," Alvarez said through an interpreter. "I feel satisfied because I gave a great fight. It was a clear victory."

Moretti, who scored last year's fight 115-113 for Golovkin, and Weisfeld both scored the 12th round for Alvarez, which gave him the victory and ended Golovkin's run as the longest active world titleholder. He had held a world title since 2010.

"I can't complain. That's what we have the judges for," Abel Sanchez, Golovkin's trainer, said. "We had a great fight, the one we expected the first time around. I had it close going into the 12th round. We had good judges who saw it from different angles. I can't complain about the decision, but it's close enough to warrant a third fight. Canelo fought a great fight. Congratulations."

play 2:11 Atlas calls Canelo's win 'a robbery' Stephen A. Smith and Teddy Atlas get emotional after Canelo Alvarez defeats Gennady Golovkin.

Golovkin didn't do a postfight interview with HBO and immediately left the ring. He went to the dressing room and received eight stitches for a cut over his right eye that he suffered late in the fight, and later said, "I'm not going to say who won tonight, because the victory belongs to Canelo, according to the judges. I thought it was a very good fight for the fans and very exciting. I thought I fought better than he did."

The rematch had been scheduled for May 5 but was canceled when Alvarez failed two drug tests for the performance-enhancing drug clenbuterol and was suspended for six months by the Nevada State Athletic Commission. The sequel was eventually renegotiated and rescheduled for Saturday night, but the failed tests and brutal negotiations to put it back together led to deep animosity between the fighters, who refused to even face off for photos during the buildup, including at Friday's weigh-in.

Golovkin and Sanchez spoke harshly about Alvarez, and Golovkin accused Alvarez of using PEDs for their first fight (though he never failed a test) and said he was using on purpose in the lead-up to the rematch. Alvarez said the positive tests were the result of his eating contaminated meat in Mexico, where that has been a long-standing issue.

Nonetheless, the once-friendly rivals became bitter enemies, turning the rematch into not only a meeting of two of boxing's elite fighters but also a good old-fashioned grudge match.

Alvarez (50-1-2, 34 KOs), 28, regained the middleweight title he had previously vacated, but it wasn't easy. Golovkin had some big rounds early and late, though it was Alvarez who edged many of the middle rounds.

Statistically, the fight was also very close in terms of landed shots. According to CompuBox, Alvarez landed 202 of 622 punches (33 percent), and Golovkin landed 234 of 879 (27 percent). Golovkin outlanded Alvarez in eight of the 12 rounds.

ESPN Stats & Information

Moments into the first round, the crowd began to chant "GGG! GGG!" only to be drowned out by chants of "Canelo! Canelo!" as the fighters began at a measured pace looking to establish their jabs. Golovkin got his working better to clearly win the round.

Alvarez began to let his hands go in the second round and landed a clean left hook midway through the round. But Golovkin didn't budge from the punch or from another good right hand later in the round.

Golovkin (38-1-1, 34 KOs), 36, a Kazakhstan native fighting out of Santa Monica, California, ripped Alvarez with an uppercut in the fourth round and continued to fire away upstairs and to pump a stiff jab that landed often.

Alvarez landed a booming overhand right in the fifth round, but again Golovkin didn't budge. Alvarez stalked forward looking to land another big bomb, but Golovkin continued to stick his jab in his face to keep him at bay. Alvarez backed up Golovkin continually in the sixth round with his body attack.

Alvarez, typically a counterpuncher, applied great pressure to Golovkin in the middle rounds and landed several good body shots. Sanchez even told Golovkin in the corner that he was losing the fight after the seventh round.

Golovkin didn't seem to have the usual snap on his power shots, and he backed up a lot more than usual, perhaps a sign that he is an aging 36-year-old fighter.