Serena fights way into US Open quarters as Thiem sails through

Williams, chasing a record-equalling 24th Grand Slam singles title, had to summon up every last ounce of strength to depose 15th seed Maria Sakkari in three, hard-fought sets.

The 38-year-old American needed almost two and a half hours to dislodge her 25-year-old Greek counterpart 6-3, 6-7 (6/8), 6-3 inside an empty Arthur Ashe Stadium in New York.

"She's such a good competitor. It was a really intense match," said Williams, who had been dumped out of a US Open tuneup tournament by the same opponent last month.

"I felt like she almost played better today. I was a little fatigued last time and had some cramps. Just felt like I was able to compete longer," Williams added.

There was little separating the players throughout the energy-sapping contest, with Williams notching 107 total points to Sakkari's 99.

Williams won 72 percent of points off her first serve while Sakkari won 73 percent. Sakkari also outdid Williams on aces, by 13 to 12.

Williams rallied from two games down in the deciding set, pumping herself up with loud screams that echoed around the spectator-free arena as she broke Sakkari twice to clinch the last-eight spot. 

"I'm super passionate. This is my job. This is what I wake up to do. This is what I train to do 365 days of the year," Williams said afterwards.

The American will play Tsvetana Pironkova for a place in the semi-finals after the Bulgarian ousted France's Alize Cornet in another draining three-setter.

- 'Absolutely unreal' -

Pironkova, playing her first tournament in three years after a lengthy break in which she gave birth to her son, prevailed 6-4, 6-7 (5/7), 6-3 at the Louis Armstrong Stadium.

"It's absolutely unreal, I really can't believe it," said Pironkova, whose last tournament appearance before this year's US Open was the 2017 Wimbledon championships when she exited in the second round.

In the men's draw, Thiem raced into the round of eight with a three-set obliteration of Canadian rising star Felix Auger-Aliassime. 

Thiem dismantled the 20-year-old 15th seed 7-6 (7/4), 6-1, 6-1 in just 2hr 7min.

"I played a great match today, the best one from my whole America trip," said Thiem.

Auger-Aliassime, ranked 21st in the world, can be proud of a breakout performance at the tournament, reaching the last 16 of a Slam for the first time.

Thiem will play Australia's 21st seed Alex de Minaur for a spot in the semi-finals after the Australian breezed past unseeded Canadian Vasek Pospisil 7-6 (8/6), 6-3, 6-2, in 2hr 17 min.

The competition was blown wide open on Sunday after world number one Novak Djokovic was thrown out for accidentally hitting a line judge after smashing a ball in frustration during his last-16 match.

His departure means the US Open will this year produce a first-time Grand Slam winner in the men's competition.

Also vying for a maiden Slam is third seed Russian Daniil Medvedev who takes on American Frances Tiafoe later Monday.

In the women's draw, second seed Sofia Kenin looks to join Williams in the last eight by ousting 16th-seeded Belgian Elise Mertens.

Williams is seeking to equal Margaret Court's record for the most women's Grand Slam singles titles.

A tournament victory at the weekend would also see her break away from Chris Evert, on six titles, as the most decorated US Open champion of the women's game in the modern era.