Sloane Stephens returns to Elise Mertens during the fourth round...

Sloane Stephens returns to Elise Mertens during the fourth round of the U.S. Open at the USTA Billie Jean King National Tennis Center on Sunday. Credit: Kathleen Malone-Van Dyke

Defending champion Sloane Stephens just keeps sailing along. Even, as she says, she’s not defending.

She made her way into the quarterfinals of the U.S. Open with a 6-3, 6-3 win over Elise Mertens on Arthur Ashe Stadium on Sunday night that was a bit more difficult than the score indicated, but it was a solid effort.

Stephens takes the tack that she isn’t exactly defending a title. “Defending a title isn't defending,” she said. “If you were to play the same people in a row again and beat all of them again, that would be defending your title. When you're playing a whole new draw, different seedings, different ranking, there's so much that goes into it. When you go to a tournament, it starts all over again. I think I try not to put too much emphasis on defending the title, just try to come in and win as many matches as possible.”

She will now face Anastasija Sevastova in the quarterfinals on Tuesday. Last year on her run to the Open title she had to dig out a win in the quarterfinals against Sevastova when down 1-3 in the third set.

There was some start and stop to the first set Sunday night with Stephens going up a break in the fourth game, giving it right back in the fifth, then getting it back in the eighth and holding for 6-3. It looked good when she broke Mertens to start the second, and wouldn’t you know it, she gave it right back in the second.

The shot of the night came in the fifth with Mertens serving at 40-15. Mertens lobbed Stephens deep into deuce court, but Stephens, in full sprint, ran it down and hit a magnificent cross-court forehand winner with the stunned Mertens racing to the net. Mertens held, but not for long.

In the seventh game Mertens balky forehand came undone. She made three errors with  and Stephens belted a forehand winner for the break and a 4-3 lead.

She held comfortably in the eighth and broke Mertens for the match in the ninth, smacking another cross-court forehand.

Stephens and Mertens met for the first time 17 days ago in Cincinnati, with Mertens winning in two sets. The momentum didn’t quite carry over.

“I think she played a really good match. I had to dig a little bit deeper to win a point,” Mertens said. “All credits to her, but I think I made a little too much mistakes.”

Karolina Pliskova defeated Ashleigh Barty, 6-4, 6-4, to book a spot in the quarterfinals against Serena Williams. Pliskova defeated Williams in the semifinals in 2016 before losing to Angelique Kerber in the final.

“It will be, for me, a dream to play again on Arthur Ashe, for sure, and to play Serena, because obviously last match we played, I won,” Pliskova said. “Since then, we didn't play. A lot of things happen between. That will be, I think, a match where I want to repeat on Arthur Ashe where we played in 2016.”

It hasn't been an outstanding season for Pliskova, who has one tournament victory, but a positive outlook carries her forward.

“Look, I'm not playing the best tennis this year, but somehow on the Grand Slam, I was just able to find my mentality was working,” she said. “So that's why I win a lot of those matches which are not great, not easy. I'm just able to beat those girls and just to go through somehow.”

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