Reigning U.S. Open champions, Sloane Stephens and Rafael Nadal, keep defense alive

Sunday was a good day for the defending U.S. Open women’s and men’s champions, who advanced to their respective quarterfinals. Sloane Stephens, the No. 3 women’s seed, overpowered No. 15 Elise Mertens of Belgium 6-3, 6-3 at Arthur Ashe Stadium, a few hours after No. 1 men’s seed Rafael Nadal pushed past unseeded Nikoloz Basilashvili of Georgia 6-3, 6-3, 6-7 (6), 6-4.

Stephens, who lives in Florida, was grateful for the crowd’s passionate support. “It’s crazy. It’s so special,” said Stephens, who next will face Anastasija Sevatova of Latvia, a three-set upset winner over Elina Svitolina. “I think an American playing at the U.S. Open is incredible. But obviously winning here, being the defending champion is crazy. Then to follow that up and do well again, I think it’s something to really get behind.”

Nadal said he made things difficult for himself with some poor shot selection early in the third set but was happy to still be standing. He will face No. 9 seed Dominic Thiem of Austria, who upset 2017 U.S. Open runnerup and No. 5 seed Kevin Anderson 7-5, 6-2, 7-6 (2). “Now is the moment to make the next step, step forward, play more aggressive,” Nadal said. “I did a lot of things well during the whole season. Is the moment to make it happen again. I hope to be ready to make that happen.”

This will be Thiem’s first appearance in the U.S. Open quarterfinals. He’s 3-7 against Nadal in his career, including this year’s French Open final. All 10 matches were played on clay, where Nadal reigns. “On clay, I think it’s one of the biggest challenges in sports to beat this guy or to compete with this guy,” Thiem said. “I hope that it’s a little bit more comfortable on hard court, but I’m not sure.”

John Isner, the top-seeded American man at No. 11, served 20 aces in outlasting Milos Raonic of Canada 3-6, 6-3, 6-4, 3-6, 6-2. Isner, 33, said he has improved his overall game and is mentally tougher, traits that helped him win a major tournament in Miami and reach the Wimbledon semifinals this year. “I’m feeling really good. Just in a good place, happy,” said Isner, whose wife remained home in Texas because she’s due to give birth to the couple’s first child on Sept. 22. “I’m really enjoying competing, not putting too much pressure on myself. To do that at 33 now is pretty cool.”

Isner will face No. 3 seed Juan Martin del Potro of Argentina, who defeated No. 20 Borna Coric 6-4, 6-3, 6-1 in the day’s final match and has yet to lose a set. This is the third straight year del Potro has reached the quarterfinals here and sixth overall. He won the title in 2009 and has always been a fan favorite among the many Argentines who attend as well as with Americans. “I love to play with this energy from the crowd, the people. They make me feel special, to play my best tennis every year,” said del Potro, whose 7-4 lifetime edge over Isner includes a round of 16 win at the French Open this year.

Late, late show

In the city that never sleeps, 2014 U.S. Open champion Marin Cilic and 19-year-old Australian Alex de Minaur worked very late Saturday night. In a 4-hour match that began Saturday and ended at 2:22 a.m. on Sunday, de Minaur saved seven match points —including being down 0-40, 2-5 in the fifth set — before Cilic prevailed 4-6, 3-6, 6-3, 6-4, 7-5 in front of an appreciative crowd at Louis Armstrong Stadium.

“Every time I step out on court, I tell myself that I’m going to play until the last point. He was going to have to beat me. Eventually that’s what he did,” de Minaur said. Cilic praised de Minaur’s spirit and had reason to be proud of himself. “Even though I missed so many opportunities there to finish it off I stayed very focused. I kept believing in my game. Came up with some great shots in the last game,” said Cilic, who will face No. 10 David Goffin on Monday in the fourth round.

helene.elliott@latimes.com

Follow Helene Elliott on Twitter @helenenothelen