Zavec (31 wins, 18 KO, 2 losses) pulled out of what had been a close-fought match after his right eye closed up from a nasty cut to the eyelid sustained during a clinch in the fifth round.

Returning from the first loss of his career, Berto (28 wins, 22 KO, 1 loss) was the more aggressive fighter out of the blocks, immediately going to work with his powerful uppercut on his more cautious opponent.

But the 35-year-old Slovenian looked to be hitting his stride, landing several right-handed hooks and jabs in the fourth round, just before suffering the injury that prevented him from fighting on.

Zavec's camp worked feverishly to close the cut during the break, but to no avail, prompting the ringside doctor to order a halt to the fight.

"It's a shame I was unable to show everything. I was saving my energy for the second half of the fight, but then the injury happened, which blurred my vision," said the Slovenian after the fight.

"Until then everything had been under control and I'm sure I was ahead on points," Zavec told the Slovenian press.

Although suspecting that the injury was caused by a headbutt by Berto, Zavec said that he had nothing bad to say about the fight.

"Berto was very fast, but I expected him to be even faster. He justified his reputation, while I am hoping and wishing for a rematch."

The German Press Agency reported that Berto indeed offered a rematch to Zavec immediately after the fight.

"We hope that Berto will keep his word. The fight was decided by an injury...It was hard for us to break off the fight, but the physical wellbeing of the athlete always comes first," Zavec's manager Uli Steinforth said.

Berto also said that he wanted to come out and make a statement following his painful defeat to countryman Victor Ortiz in April. "I think it was my speed that made the difference tonight," he said.

Berto added he was ready and hungry for more success, notably also against Ortiz who is sure he will meet in the ring again.

Zavec had held the International Boxing Federation's welterweight belt since December 2009, when he won it with a third-round knock out of South African Isaac Hlatshwayo in Johannesburg.

He had mounted three successful title defences at home prior to today's highly-anticipated fight at the Beau Rivage Resort & Casino, which was his first fight in the US and only his second outside of Europe.

Zavec indicated he would take some time to decide on the future course of his career now. Berto will meanwhile look for a rematch with Ortiz.