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"I came out today and probably felt the best I've felt in a few months. Was feeling the ball incredibly well from both sides," Zverev, who won the Cincinnati title in 2021, said.

But the German struggled physically and called for the doctor in the second set.

"In the first set I started feeling not so great and it got progressively worse. But I'm in the semi-finals and I'll do everything I can to be 100 per cent tomorrow."