Serena Williams, Thiem advance at US Open

Williams's ambition of securing a record-equaling 24th Grand Slam title remains alive as she moved into the women's quarterfinals after an immensely hard-fought 6-3, 6-7 (6), 6-3 victory over Maria Sakkari of Greece, while Thiem had a much easier time with a 7-6 (4), 6-1, 6-1 win against Canadian rising star Felix Auger-Aliassime in the men's draw on Monday.

Sakkari threatened to repeat her come-from-behind victory over Williams in the Western & Southern Open two weeks ago, but ultimately fell just short.

"It's a completely different match and a completely different scenario, a completely different moment. She was doing so well, she was being so aggressive, and I knew I needed to do the same thing," Sakkari said.

Williams will take on Bulgaria's Tsvetana Pironkova, who overcame Alize Cornet of France 6-4, 6-7 (5), 6-3 in another round of 16 match.

Belarus's Victoria Azarenka advanced into the quarterfinals after rallying past Karolina Muchova of the Czech Republic 5-7, 6-1, 6-4.

Elsewhere, Belgium's Elise Mertens upset second seed Sofia Kenin of the United States 6-3, 6-3 to also make the last eight.

"I played pretty aggressively from the first ball, and that's pretty necessary against her because she's a great player who can hit a lot of great angles," Mertens said. "I tried to step into the court, and thankfully my first serve was working, so that was a big advantage."

Kenin has won her last 10 major main draw matches after hoisting her first Grand Slam title at this year's Australian Open, but the Belgian proved too strong for her, advancing after just 74 minutes.

In the men's singles, Russian players Daniil Medvedev and Andrey Rublev joined Thiem in the last eight while Australia's Alex De Minaur also advanced.