Angelique Kerber cruises into last eight

The German made her first major semi-final at Flushing Meadows five years ago, but this season came an even bigger breakthrough when she beat Serena Williams for the Australian Open title. Kerber could overtake Williams for the No. 1 ranking depending on their U.S. Open results.

Kvitova hadn't made it past the third round at the first three majors of the year but was coming off an Olympic bronze medal in Rio de Janeiro, where Kerber won the silver.

The 14th-seeded Kvitova had 43 unforced errors and seven doubles-faults - the last coming on match point.

Caroline Wozniacki, derailed this year by an ankle injury which sent her career into a tailspin, and Anastasija Sevastova, who quit the sport three years ago, set up a quarter-final duel.

Former world number one Wozniacki, the runner-up in 2009 and 2014, downed American eighth seed Madison Keys 6-3, 6-4 to make the last-eight in New York for a fifth time yesterday.

Now ranked at 74 in the world after a right ankle injury sidelined her for almost three months, the Danish star hit just seven unforced errors in her win over Keys.

Wozniacki is back in the last-eight of a Slam for the first time since making the semi-finals in New York in 2014.

"After such a tough year, it's amazing to be in the quarter-finals," said the 26-year-old whose Grand Slam record this year had seen first round losses in Australia and Wimbledon while she sat out Roland Garros.

"I was nervous in the last game. I told my serve please hold out. I knew Madison wouldn't give much up and that she'd go for the big shots, so I tried to keep it on her backhand side."

Sevastova became the first Latvian woman in 22 years to reach a Grand Slam quarter-final when she beat British 13th seed Johanna Konta 6-4, 7-5. The 26-year-old world number 48 knocked out French Open champion and third seed Garbine Muguruza in the second round and she built on that victory on Arthur Ashe Stadium in a last 16 tie which featured 12 breaks of serve.

The Briton was undone by 34 unforced errors. Larisa Savchenko was the last Latvian woman to make the quarter-finals of a Slam at Wimbledon in 1994.

Sevastova had not won a match in New York since 2010 before this year and with her career unravelling she quit in May 2013 to study leisure management in Austria.

Wozniacki defeated Sevastova in their only previous meeting, in straight sets in the Australian Open last 16 in 2011.

Italian seventh seed and 2015 runner-up Roberta Vinci reached the quarter-finals for the fourth time with a 7-6 (7/5), 6-2 win over Lesia Tsurenko of Ukraine.