Victory in the all-German fight at the Lanxess Arena gave Sturm a 37-2-2 record with 16 knockouts and the 33-year-old has now not lost a fight since 2006, when he fell to Javier Castillejo.
Zbik, who started the brighter of the two fighters on the night but tired midway through the bout, dropped to 30-2 (10 KOs).
Sturm had his opponent in trouble at the end of the ninth round, prompting American referee Raul Caiz Sr. to ask the challenger if he was able to carry on.
He said he could, but his corner thought differently and the fight was stopped before the 10th 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 said: "I have to admit without any grudges that Felix clearly beat me."