Zbik made the better start at the Lanxess Arena but began to tire midway through the bout, with the defending champion looking fresher despite taking some heavy blows early on.

Sturm had his fellow German in trouble 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 that he could, his corner felt otherwise and the match was stopped before the next round could start.

"We fought a fair fight. It was a good advertisement for boxing," Sturm said. "Sebastian's a great boxer, he moved very well. I had to keep my pace. It took a while and it was one of my hardest fights but thank God it worked out."

Zbik (30-2) added: "I have to admit without any grudges that Felix clearly beat me."

The 33-year-old Sturm improved his record to 37-2-2 with 16 knockouts.