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Zverev appeared to injure his right toe when serving for the match at 5-4 in the second set, and he was treated by the physio after the first point of the game.

It did not stop the world No.3 from comfortably serving out for his win, but he later acknowledged feeling uncertain about how serious a problem it could be.

“I’m very happy [to get through],” the German said.

“I don’t know, I landed funny on my toe and after that I could barely do a step, so we’ll see what it is.

"Not happy to end the match like that, of course, but I think definitely happy with the level in the first round of a Masters.

"That was quite good. He is playing very well and moving forward, we will see if I will ever be healthy this year again, because it’s been a struggle. But I’m happy to be through.”