Juan Martin del Potro advanced in straight sets to the semi-finals of the rain-hit Cincinnati Open tennis tournament.

Del Potro beat France’s Jeremy Chardy 6-1 6-3 in the opening match on Friday morning, reaching the tournament’s semi-finals for the first time. His biggest concern has been a sore left wrist that he’ll have examined before the US Open.

The Argentine won the Open in 2009 but the right-hander missed most of the 2010 season while recovering from surgery on his right wrist. The doctor who performed that surgery will look at the other one following this week’s tournament.

“The good thing: I’m not getting worse, so I can play,” he said. “But anyway, I want to wait (for) what the doctor says because I have experience on my other wrist.”

Del Potro won the bronze medal at the Olympics in London by beating Novak Djokovic, then was knocked out in the first round of the Rogers Cup in Toronto last week. He’s still trying to get his game back to a championship level.

“I’m still feeling a little difference between the top players and me,” he said. “But I know I’m getting closer very slowly, so that’s important. I’m working for that.”

Rain interfered with two singles matches on Thursday night.

Sixth-seeded Caroline Wozniacki was down a set to Russia’s Anastasia Pavlyuchenkova when the rains came. Pavlyuchenkova finished off a 6-4 6-4 win on Friday, then had eight hours to rest before playing a quarter-final match against fourth-seeded Petra Kvitova.

Ninth seed Li Na’s match against Sweden’s Johanna Larsson was postponed before they could take the court on Thursday night. Li needed only 64 minutes to win 6-2 6-2 on Friday morning, leaving her less than seven hours to get ready for her quarter-final match against top-seeded Agnieszka Radwanska of Poland.