The 81st-ranked Riske won 6-3, 6-0 on Saturday, taking the last eight games against the seventh-seeded Czech.
Riske had been 0-5 at major tournaments before Wimbledon this year but is 5-1 since.
The 23-year-old Pittsburgh native just broke into the top 100 in late July; now she’ll likely earn a top-60 ranking.
This was Riske’s first victory over a top-10 opponent.
“I’ve got a new confidence in myself,” Riske said through tears in an on-court interview. “I believe that I belong here.”
Kvitova called for a trainer before the final game. She has always struggled at Flushing Meadows, the only major tournament at which she hasn’t made the semifinals. Kvitova had seven double-faults and 27 unforced errors.
Riske has always thrived on grass, making the third round at Wimbledon this year. Now she’s starting to figure out the hard courts.
“It’s really tough out here,” she said. “Every week isn’t like this.”
SPANIARD CRUISE: Rafael Nadal crushed another opponent at the U.S. Open, beating Ivan Dodig 6-4, 6-3, 6-3.
The second-seeded Spaniard has lost just 21 games through three matches.
He has faced only five break points, saving each one of them.
The 38th-ranked Dodig beat Nadal at the hard-court tournament in Montreal two years ago. But Nadal is looking like the king of hard courts these days, with an 18-match winning streak on the surface.
He closed out the victory with a service winner to win the last game at love.
EX-BULLDOG BOUNCED: Despite 26 aces and plenty of home-crowd support, unlike in his previous match, 13th-seeded John Isner, a former Georgia standout, lost to 22nd-seeded Philipp Kohlschreiber in the U.S. Open’s third round for the second consecutive year.
Kohlschreiber’s 6-4, 3-6, 7-5, 7-6 (5) victory Saturday over the highest-ranked American means the host country has only one man left: 109th-ranked Tim Smyczek, whose third-round match is today.
After beating France’s Gael Monfils in the second round, Isner said he was “disappointed” that fans were cheering so vociferously for Monfils.
On Saturday, there were chants of “U-S-A! U-S-A!” in the second set, and “Here we go, John! Here we go!” in the fourth set – but Isner failed once again to figure out Kohlschreiber at Flushing Meadows.