A confident Alison Riske upsets ailing Petra Kvitova at U.S. Open

NEW YORK — Alison Riske, an unseeded 23-year-old from Pittsburgh, showed no nerves and had plenty of big ground strokes in upsetting seventh-seeded and former Wimbledon champion Petra Kvitova, 6-3, 6-0, in a third-round match at the U.S. Open on Saturday.

Unassuming German Philipp Kohlschreiber, seeded 22nd, for the second straight year, upset 13th-seeded American John Isner in the third round. Isner fell 6-4, 3-6, 7-5, 7-6 (5).

Riske is part of a large group of young American women making their presence felt at this Grand Slam event, including 20-year-old Sloane Stephens, who will play top-seeded Serena Williams on Sunday in the fourth round.

There was also 17-year-old Victoria Duval, who eliminated former U.S. Open champion Samantha Stosur in the first round; 23-year-old Jamie Hampton, who lost to Stephens on Friday; and 21-year-old Christina McHale who played 13th-seeded and former world No. 1 Ana Ivanovic tough before losing, 6-4, 5-7, 6-4, in 2 hours 20 minutes Saturday.

A young Italian woman made some noise Saturday night. Qualifier Camila Giorgi, 21, ranked No. 136 in the world, took 2009 U.S. Open finalist Caroline Wozniacki, a 23-year-old who was once ranked No. 1 in the world, the distance before beating the sixth-seeded Wozniacki, 4-6, 6-4, 6-3, in a third-round match.

“It was really emotional for me,” Riske said of the win over Kvitova. “It’s just really cool because I feel like I’m playing within myself. I’m not doing anything crazy. It just feels really comfortable. It’s exciting.”

Riske celebrated by hitting balls into the stands and squealing with excitement.

Kvitova, who suffers from allergies, always has trouble keeping up her stamina on U.S. hard courts.

McHale, from Teaneck, N.J., who stays at home during the Open, saw her ranking drop below 100 this year after she missed much of last season with mononucleosis. McHale has said it’s taken her almost until now to rebuild her strength and stamina.

Kvitova said it was more than allergies that bothered her Saturday.

“Unfortunately, I was lying in bed yesterday and I had a fever, so I didn’t come to the site Friday,” Kvitova said. “I didn’t even come to the site. My body didn’t help me today to move a little bit. Unfortunately I tried to play, tried to fight. But my body wouldn’t let me fight.”

Second-seeded Victoria Azarenka lost a first set to Frenchwoman Alize Cornet. But Azarenka recovered for a 6-7 (2), 6-3, 6-2 win.

Azarenka seemed cranky all day and had a testy exchange with the chair umpire at one point.

Serving in the second set for a game, Azarenka pumped her fists and walked off the court after the 26th-seeded Cornet hit a backhand into the net. The chair umpire had the unhappy duty of telling Azarenka her previous shot had been called out.

“Are you … kidding me?” Azarenka said. “What … are you doing?”

Winning in much less dramatic, and friendlier fashion, was men’s No. 2 seed Rafael Nadal. The Spaniard, who is on course to meet Roger Federer in the quarterfinals, earned his third straight three-set win, beating Ivan Dodig 6-4, 6-3, 6-3.

“I am winning because I am playing well from the baseline and I am making the right decisions at the right moment,” Nadal said.

Five-time U.S. Open champion and seventh-seeded Federer overpowered Frenchman Adrian Mannarino in Saturday’s final match, winning 6-3, 6-0, 6-2. Nadal and Federer have played 31 times but never at the U.S. Open.

Federer will play Tommy Robredo in the fourth round and Nadal gets Kohlschreiber.

Isner, who had expressed disappointment Thursday when the American crowd seemed to plant itself on the side of his French opponent Gael Monfils, was only disappointed in himself Saturday.

“I was trying to get myself fired up,” Isner said. “When it gets like this in a match going pretty long, the conditions were rough, it was so muggy out there. I was struggling with that a little bit.

“But, really, I didn’t have much left. If I could have held on, not gotten broken there at the end of the fourth set, I would have liked my chances in the fifth. But in that tiebreaker, I was pretty gassed.”

diane.pucin@latimes.com