The world No 6 won all of her first-serve points in the first set and dropped just one in the second, as Kvitova struggled with her serve, producing five double-faults across the 66-minute affair in California.

Two-time Wimbledon winner Kvitova ceded breakpoint in the eighth game of the first set and never regained her composure, as Sakkari fended off two breaks to close out the frame and ran away with the match in the second set.

“It was a little bit tough for me to get used to her serve,” said Sakkari, who reached the semifinal at Roland-Garros and the US Open last year.

“Once I broke her, I just felt like that’s was the right way to return her serve and I was just very solid from the back of the court.

It was the second time that Sakkari has reached the round of 16 at tennis’ unofficial “fifth major,” after she reached the fourth round in 2018.

Later in the day’s action, two-time winner and 13th seed Victoria Azarenka fell to Kazakhstan’s Elena Rybakina 6-3, 6-4 after a malfunctioning serve saw her produce eight double faults.

An in-form Rybakina produced a pristine first set, launching four aces without dropping a single first-serve point to claim the early momentum.

With her right shoulder heavily taped and a sleeve placed on her right arm, two-time Australian Open champion Azarenka grimaced, crouched down on the court and wept after a double fault in the fifth game of the second set.

That prompted the chair umpire to approach Azarenka to ask if she needed medical attention.

Azarenka, who lost in the final last year to Spain’s Paula Badosa, apologised and continued but Rybakina converted the critical breakpoint in the seventh game of the second set.

Elsewhere on the women’s side, Czech Marketa Vondrousova knocked out Estonian fourth seed Anett Kontaveit 3-7, 7-5, 7-6(5).