Former world champion Sergey Kovalev admits the gas tank was running low after the sixth round of Saturday's title fight with Mexican superstar Canelo Alvarez.

Canelo, who was moving up by two full weight divisions to challenge Kovalev for his WBO light heavyweight title, knocked the Russian fighter out in the eleventh round at the MGM Grand in Las Vegas.

Kovalev was coming off a tough eleventh round knockout over unbeaten Anthony Yarde - which took place on August 24th in Russia.

According to Kovalev, his stamina took a real hit because he returned to soon to the ring.

The bout with Canelo guaranteed him a career-high financial package worth $12 million - too good of an offer to turn down.

But he explained that a tough training camp for the grueling fight with Yarde - and then heading right back to training camp for Canelo - was too much for his body to handle at 36-years-old.

“I was tired after round six, because I had my last fight very close to this one, but it’s okay, it’s a new experience for me. Canelo is really a great champion. A little bit right now, I didn’t recover from my last fight. But it’s okay. Thanks for the fight Canelo, I have big respect for him. He made history," Kovalev said.

“To open my body less, I had to use more jabs, more safe. I’ll be back. I’ll be back much stronger.”

Kovalev would also explain that his inability to secure unification fights was the reasoning for holding out until the Canelo fight was finalized.

“I know that I can be a unified champion, let’s make a unification fight - but nobody wants to fight me in my division," Kovalev said. "This guy [Canelo] is more fresh, he came into my division more fresh. He has more stamina, but it’s okay, it’s a good experience for me.”