Kazakh southpaw Janibek Alimkhanuly stopped Germany's Vincenzo Gualtieri in the sixth round on Saturday to unify two middleweight world boxing titles in a showdown of unbeaten champions.

Alimkhanuly kept the World Boxing Organization (WBO) title and took the International Boxing Federation (IBF) crown from Gualtieri when referee David Fields stopped the fight in suburban Houston after one minute and 25 seconds of the sixth round.

"He was waiting because he was thinking I was going to get tired," Alimkhanuly said through a translator. "But I did not get tired."

Alimkhanuly improved to 15-0 with his 10th victory inside the distance while Gualtieri, fighting outside Germany for the first time, fell to 21-1 with one drawn.

Alimkhanuly was defending his WBO throne for the third time and took the IBF title Gualtieri was defending for the first time after winning it from Brazil's Esquiva Falcao in July.

"We would like to add another two belts to this collection," Alimkhanuly said, staking his case for a chance at an undisputed crown.

The other world champions in the 160-pound division are Americans Jermall Charlo for the World Boxing Council and Erislandy Lara for the World Boxing Association.

The 30-year-old fighters measured each other in the early rounds before Alimkhanuly began pressing the attack, his stinging jabs and power punches keeping Gualtieri at bay.

Alimkhanuly landed a stunning left uppercut to the chin in a dominant fifth round.

"This is my style of fighting. This is Kazakh style. This is what I'm going to continue doing," Alimkhanuly said.

Gualtieri was staggered with another left uppercut early in the sixth, followed by a series of hard jabs and more hammering lefts that prompted Fields to stop the fight.

"I think it was a little bit early to break the fight but surely he was better today," said Gualtieri, who was sporting a swollen left eye.

"He had a good punch, a hard punch. We see that on my face. But I think it was a little bit early."

Alimkhanuly was a 2013 world amateur champion, 2014 Asian Games gold medalist and 2016 Olympian.