NEW YORK—Petra Kvitova, who lost to Flavia Pennetta in the quarterfinals at the U.S. Open, says she thinks that Serena Williams will win the tournament.

The two-time Grand Slam champion is one of only two players to have beaten Williams this year, defeating her in Madrid before 18-year-old Belinda Bencic upset the world No. 1 in Toronto. Williams will face Roberta Vinci in the semifinals.

“I saw the match Serena played yesterday with Venus, and I just felt that she's more focusing than she did before, the matches before, and I think that she's kind of the player who with the tournament she's playing better and better every match,” Kvitova said. “So I think that if she gonna be in the final, I think she gonna put everything what she has inside and she gonna win it.”

Kvitova added that she was not 100 percent healthy against Pennetta. The Czech discovered that she had mono right after Wimbledon and has said that it could take months before she is fully healthy again. The world No. 4 played and won New Haven right before the U.S. Open.

“I'm disappointed that I lost today for sure, but on the other side, I think I have a lot of positive things to take from the tournament," Kvitova said. "It wasn't really easy situation. I'm kind of proud of myself how I played here, anyway. I didn't really practice that much before. Everything was just like puzzles to take it together somehow and just take it together somehow and everything.”