Serena Williams and Sloane Stephens are through to a highly anticipated matchup in the round of 16 at the U.S. Open, where they're joined by a less expected American, Alison Riske.

Maybe's Riske's run isn't that much of a surprise considering her recent surge. The 23-year-old from Pittsburgh came into the summer having never accomplished any of these feats: winning a match in a Grand Slam tournament, at a WTA Tour event on hard courts, or against at top-10 foe.

She's now achieved all that, her latest breakthrough victory a rout of 2011 Wimbledon champ Petra Kvitova on Saturday. Riske won 6-3, 6-0, taking the last eight games against the seventh-seeded Czech, who was in bed with a fever the day before.

"I've got a new confidence in myself," Riske said through tears in an on-court interview. "I believe that I belong here."

She was 0-5 at major tournaments before Wimbledon this year but is 5-1 since. Riske just broke into the top 100 in late July; now she'll likely earn a top-60 ranking.

"It's really tough out here," she said. "Every week isn't like this."

She has always thrived on grass, making the third round at Wimbledon this year. Now she's starting to figure out the hard courts.

Kvitova said she tried to end points quickly, knowing she couldn't hold up through long rallies. But Riske stayed calm and played good defense, taking advantage of Kvitova's seven double-faults and 27 unforced errors.

"She moved quite well," Kvitova said. "She pushed me to the back. That was tough for me."

It's another frustrating finish at Flushing Meadows, the only major tournament at which she hasn't made the semifinals. Two years ago, she became the first reigning Wimbledon women's champion to lose her first U.S. Open match in the same season.

"My body wouldn't let me fight," Kvitova said.

Riske was all set to play for Vanderbilt in 2009 when a family friend who owns a chemical company offered to sponsor her. So she turned pro.

A year ago at this time, she was questioning the wisdom of that decision. Then she rejoined coach Yves Boulais.

"Once I got back with him, things kind of unfolded themselves," Riske said. "I felt really comfortable. I knew that with the tennis I was playing that things were going to start coming together. I just didn't know when."

Williams advanced after midnight Saturday morning, beating 78th-ranked Yaroslava Shvedova of Kazakhstan 6-3, 6-1. The previous match in Arthur Ashe Stadium on Friday night lasted more than four hours: 2001 U.S. Open champion Lleyton Hewitt's 6-4, 5-7, 3-6, 7-6 (2), 6-1 comeback victory over 2009 champion Juan Martin del Potro.

Much, much earlier, Stephens reached the round of 16 in New York for the first time by beating 23rd-seeded Jamie Hampton 6-1, 6-3 in another all-American matchup.

Ever since Stephens announced her presence as a major force with a quarterfinal victory over a hobbling Williams at the Australian Open, tennis fans have craved a rematch on the biggest stage. Now it's here.

Saturday's session opened with two minor upsets: Simona Halep, seeded 21st, crushed No. 14 Maria Kirilenko 6-1, 6-0, and Flavia Pennetta beat 27th-seeded Svetlana Kuznetsova 7-5, 6-1.