Gilles Simon, who lost to Stan Wawrinka in the fourth round at Roland Garros, says the Swiss can be a very formidable though sometimes unpredictable opponent. The 2014 Australian Open champion will face Roger Federer in the quarterfinals.

“He played such a high level of tennis that day,” Simon said. “Sometimes he can do that again. When you play him, you have the impression that it's always going to happen...And playing me in these conditions, that was perfectly adapted to his style, to his game, and I didn't manage to put enough stress on him."

Wawrinka has won four clay-court titles in his career, including the ATP Masters 1000 in Monte Carlo. This season, he won at Chennai and Rotterdam on hard courts. He reached the quarterfinals of the Australian Open but lost 6-0 in the fifth set to Novak Djokovic. He lost early at Indian Wells and Miami, where he said, “I had two difficult months.”

The 30-year-old began to feel better on clay.

“Since Madrid I played very well. I lost to Dimitrov, but it was mentally [a problem],” Wawrinka said. “Because you can't all of a sudden decide to fight at all times and win all matches. It takes a bit of time. But in Rome my tournament was good, my game was good...I know that my game level can be very high, and that when I'm confident, I can beat many tough players when I do the right things every day.”