NEW YORK (AP) — Roberta Vinci fell apart after losing the opening set on a foot fault, allowing No. 2 seed Angelique Kerber to take the last nine games and win their U.S. Open quarterfinal 7-5, 6-0 on Tuesday.

Vinci was the runner-up at Flushing Meadows a year ago, reaching her first major final by stunning Serena Williams to end the American’s bid for the first calendar-year Grand Slam in tennis in more than a quarter-century.

But after being two points from taking the first set against Kerber while serving for it at 5-4, 30-all, the No. 7-seeded Vinci faltered badly. She missed a forehand long, then netted a backhand to get broken there — and that was just the beginning of her collapse.

Trailing 6-5, and serving at love-40, Vinci missed her first serve, then was called for a foot fault on a second serve. That resulted in a double-fault, ceding the set.

As she walked to the sideline, Vinci looked at the line judge who made the call and smiled sarcastically, giving him a thumb’s up and applauding with her racket.

It’s a rare ruling in Grand Slam tennis, especially at a critical juncture, although there was, of course, the most famous foot fault of all on the very same court. In the 2009 U.S. Open semifinals, Williams was angered by the same type of call: a foot fault that resulted in a double-fault; in that instance, it set up match point for her opponent, Kim Clijsters. Williams brandished her racket and yelled at the line judge, and the point she was docked for that ended the match.

This time, the call ended the competitive portion of Vinci’s quarterfinal: She managed to take only 10 of 38 points the rest of the way.

Vinci has been dealing with an injured left Achilles tendon — she wore black tape in the shape of a “V” that framed her left calf — and a bad back. Still, her varied game, filled with slices and drop shots and net rushes, gave Kerber fits for most of the first set.

“It’s always difficult to play against Roberta. She’s a great player,” Kerber said. “She played so well last year here.”

Kerber, who has a chance to overtake Williams at No. 1 in the WTA rankings after the tournament, moved into her third Grand Slam semifinal of the year. She won the Australian Open in January, then lost to Williams in the Wimbledon final in July.

Kerber’s first U.S. Open semifinal since 2011 will come against an unseeded player, either two-time runner-up Caroline Wozniacki or Anastasija Sevastova.

The men’s quarterfinals Tuesday were No. 1 Novak Djokovic vs. No. 9 Jo-Wilfried Tsonga, and No. 10 Gael Monfils vs. No. 24 Lucas Pouille.

The day’s first quarterfinal was a ragged match in many ways, even before Vinci’s sudden drop in level. Through the first six games, when the score was 3-all, there had been four service breaks and the players had combined for 26 unforced errors, accounting for nearly half of the match’s points until then.

During the first point at 4-all, with Kerber serving, a jet — LaGuardia Airport is nearby — roared its way over Arthur Ashe Stadium, creating quite a ruckus and distraction. Kerber missed a shot to lose the point, then glared overhead.

“I’m trying to improve, of course, my mental side, because I know I need to stay focused on my game, trying to be here in this moment and not thinking about what’s happened and just thinking about the positive things,” Kerber said. “Just going for it and just trying to stay a little bit relaxed when it’s close and just go for it when I have the chance.”

She would add three unforced errors in a row after the airplane interruption to get broken at love and trail 5-4. Hard to believe at the time, but she would not lose another game.