Moments later, Alvarez (24-0, 12 KOs) unleashed a combination and Kovalev hit the deck again and the fight appeared over.
Kovalev's promoter Kathy Duva confirmed there is a rematch clause, but said it's too early to say when or if the return fight would take place.
Kovalev (32-2-1, 28 KOs), 35, a Russian Federation native living in Los Angeles, hopes a victory will lead to a unification fight so he can begin reclaiming the rest of the belts he lost to Ward. At that point the referee moved in and called an end to the fight with 15 seconds left in the round.
"As the fight went on, I wanted to show that I could stay strong and do good things", Alvarez said. I will try to do my best for boxing fans, for people who watch our fight. This was all for them. "And I also didn't have a coach in my training camp".
After the fight, Kovalev was taken to the hospital to get checked out. He's unsafe. I cannot say whether I can knock him out or get a victory by points. If he tastes his punch it could be lights out so he has to fight the ideal fight to come home with the belt. He got up quickly but Alvarez dropped him with a quick and furious combination less than a minute later.
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He'll have to work to put another knockout victory to his name, however, as Alvarez is pretty technically sound and will be willing to try and slow this fight down.
"You have a guy in Eleider Alvarez who has been waiting for his title shot for over two years, and a guy in Sergey Kovalev who jumped at the chance to give Alvarez the opportunity", said Main Events matchmaker Jolene Mizzone.
It wasn't until the fourth round when Kovalev tried to exert his power, landing several hard punches that tested Alvarez's chin. Alvarez shook it off and taunted Kovalev later in the round, but an accidental head-butt caused blood to stream down the left side of Alvarez's face. Alvarez rocked Kovalev with a huge right hand that sent the champ staggering backwards and down to the mat.
According to CompuBox punch statistics, Bivol landed 154 of 447 shots (35 percent) and Chilemba landed 73 of 472 (16 percent).
Kovalev followed with a soft-touch defense against Igor Mikhalkin, whom he knocked out in the seventh round in March.
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His campaign spent $2.25 million on advertising compared to Balderson's $507,000, according to campaign tallies of ad spending. In Michigan, three mainstream Democrats in suburban Detroit were among those vying for a chance at retiring Republican Rep.
And then Andre Ward came into his life and beat him twice in a row, the first time via highly controversial decision to take his belts in November 2016.
But Kovalev never accepted or took responsibility for either of the losses, mocking Ward [whose ringname was Son of God] by calling him "Son of Judges" and made childish excuses about low blows in the second fight.
Following his second loss to Ward, Kovalev parted ways with his trainer John David Jackson and chose to go with Abror Tursunpulatov as his new trainer.
Former Middleweight champion Peter Quillin is fighting for only the second time since he was knocked out by Daniel Jacobs in one round back in 2015 whereas J'Leon Love has been living on borrowed time feasting on low level opponents over the last few years.
"I was very happy with how the fight was going but I kept saying to (Kovalev manager) Egis (Klimas), 'Let's get past the sixth round and I'll relax.' But you saw what happened".
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Kovalev's future in boxing is unknown, but Alvarez now has the world ahead of him.