PARIS—Stan Wawrinka, who won the Geneva tune-up tournament just days before the French Open, says he is finally feeling comfortable on clay—a surface that's treated him well over the years.

The 32-year-old Swiss won the Monte Caro Masters in 2014 and the French Open in 2015, but has struggled on the slow surface this season. He lost matches before the quarterfinals at Monte-Carlo, Madrid and Rome before defeating Mischa Zverev to win a home-country title in Geneva.

“Especially the fact that it's in Geneva, yes, builds my confidence," Wawrinka said. "I don't think I was really at the best of my shape when I started the tournament in Geneva, but coming here [to the French Open] I feel great.

“I feel in good shape. I think the best thing I could do was to win Geneva. I started to play much better.”

The No. 3 seed at Roland Garros had a similarly slow start on clay in 2015, doing little until Rome, where he upset Rafael Nadal before losing to Roger Federer. At Roland Garros, Wawrinka defeated Federer in the quarters in straight sets, then ousted Jo-Wilfried Tsonga and Novak Djokovic to win the tournament, his second Grand Slam title.

In 2016, Wawrinka reached the semifinals at Roland Garros before losing to Andy Murray. Later that summer, he won his third Grand Slam title at the U.S. Open, defeating Djokovic in four sets.

“The beginning, early season on clay was not good. It really had a negative impact on my confidence," Wawrinka said. "So being able to win matches was very important. It sort of was able to create some momentum and build confidence. Again, feeling confident, playing well, winning matches is great, but when you don't win matches it's very difficult to get the confidence. I think in Geneva I was finally able to find that confidence again.”