Felix Sturm retained the WBA middleweight title Friday night after fellow German Sebastian Zbik retired before the 10th round at the Lanxess Arena.

Zbik made the better start but appeared to tire midway through the bout, with the defending champion looking fresher despite taking some heavy blows.

Sturm had Zbik clearly troubled at the end of the ninth round, with American referee Raul Caiz Sr., asking the challenger if he could continue.

Though the 30-year-old Zbik insisted he could, his corner felt otherwise and Caiz ended the fight before the next round could start.

"We fought a fair fight. It was a good advertisement for boxing," Sturm said after successfully defending his title for the 12th time.

"Sebastian's a great boxer, he moved very well," Sturm added. "I had to keep my pace. It took a while and it was one of my hardest fights, but thank God it worked out."

Sturm, a 33-year-old German of Bosnian descent, improved his record to 37-2-2 with 16 knockouts.

His last defeat came to Javier Castillejo in 2006 before he won the WBA title back from the Spaniard the following year.

"I have to admit without any grudges that Felix clearly beat me," said Zbik, who dropped to 30-2 (10 KOs).