Juan Martin Del Potro beat qualifier Dmitry Tursunov 6-4, 3-6, 6-1 to reach the semi-finals of the Cincinnati Masters on Friday, then revealed he is dealing with pain in his left wrist.
The Argentine missed much of the 2010 season with a wrist problems and remains cautious.
"Yes, it bothers me a little bit, but not too much," said the 2009 US Open winner.
"I'm trying to play slices and different shots before the US Open to see how different I can play against the top guys or (try) doing different things."
But he added: "I'm okay, my knee is OK too, and that's important looking forward to New York. It's nothing dangerous. I'm always in contact with my doctor; he's taking care of me all the time."
Del Potro needed one and three-quarter hours to get past veteran Russian Tursunov, with the Argentine hammering 15 aces and breaking three times.
He will play the winner from top seed Novak Djokovic, bidding for his first title in Cincinnati, and American John Isner.
Del Potro reached the semi-finals in 2012 where he lost to Djokovic.
"This is the last tournament for me before the US Open, so it's a good thing for my preparations. I will try to play better than today."