Second seed Sofia Kenin of the United States was sent crashing out of the US Open on Monday, beaten in straight sets by Elise Mertens of Belgium.

Kenin, the highest seeded player left in the tournament and winner of the 2020 Australian Open, was bundled out of the last 16 6-3, 6-3, in 1hr 14 mins by Mertens.

Mertens, the 16th seed, will now play Victoria Azarenka of Belarus in the quarter-finals.

Kenin, who had not lost a game in her first three rounds, never got her service game together against Mertens.

The 21-year-old Moscow-born player won only 56 percent of first serve points compared to 75 percent for Mertens.

Kenin also hit three double faults, while Mertens hit none.

Mertens, ranked 18th in the world, smashed eight aces, while Kenin failed to muster any.

Serena the battler

Williams 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 opponent 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.

Pironkova rolls on

Tsvetana Pironkova for a place in the semi-finals after the Bulgarian ousted France’s Alize Cornet in another draining three-setter.

Pironkova, who is playing her first tournament in three years after a lengthy break during 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.

Azarenka finds another gear

Unseeded Victoria Azarenka came from a set down to oust 20th seed Karolina Muchova 5-7, 6-1, 6-4 in 2hr 30min.

Muchova took control of the match early and decisively. Before it was 20 minutes old, Azarenka was down 1-4, a victim of the 24-year-old’s aggressive groundstrokes and solid serving. Muchova took the set, 7-5.

Azarenka, from Belarus, has won two Australian Open women’s singles titles, in 2012 and 2013. That big-stage experience appeared to kick in as she started to hit winners and dial back on errors. Azarenka took the set, 6-1.

The Belarusian looked relaxed after clinching the set, dancing to music over the intercom after Muchova disappeared for a medical time out. The younger player returned with a wrapped left leg. And Azarenka started running her from side to side, breaking her after a nine-minute game to go up 2-1. But Muchova responded by going for broke, wincing as she hit winners—but hitting them all the same. Azarenka continued to apply pressure, pulling out the third set, 6-4.

“It was a great match,” said the winner in her on-court interview. “I had to really dig in. I hope people enjoyed watching it because I really had fun.”

All the results

Tsvetana Pironkova (BUL) bt Alize Cornet (FRA) 6-4, 6-7 (5/7), 6-3

Serena Williams (USA x3) bt Maria Sakkari (GRE x15) 6-3, 6-7 (6/8), 6-3

Victoria Azarenka (BLR) bt Karolína Muchová (CZE x20) 5-7, 6-1, 6-4

Elise Mertens (BEL x16) bt Sofia Kenin (USA x2) 6-3, 6-3

Quarter-final draw

Jennifer Brady (USA x28) v Yulia Putintseva (KAZ x23)

Naomi Osaka (JPN x4) v Shelby Rogers (USA)

Tsvetana Pironkova (BUL) v Serena Williams (USA x3)

Victoria Azarenka (BLR) v Elise Mertens (BEL x16)