WIMBLEDON, England — Coco Gauff’s unexpected but remarkable run at Wimbledon is over.

The 15-year-old American, who became the youngest player to qualify for Wimbledon’s main draw in the professional era, lost to former No. 1 Simona Halep 6-3, 6-3 in the fourth round on Monday.

“I learned a lot,” said Gauff, who is younger than many of the players in the junior competition at Wimbledon. “I learned how to play in front of a big crowd. I learned what it was like to be under pressure. I learned a lot and I’m really thankful for this experience.”

Gauff knocked out five-time champion Venus Williams in the first round, and then saved two match points in the third round to reach the second week of a Grand Slam tournament in her debut.

Playing on No. 1 Court, the second biggest stadium at the All England Club, Gauff had plenty of support from the crowd. But Halep, the 2018 French Open champion and a Wimbledon semifinalist in 2014, had the better shots. She broke Gauff five times and took advantage of 29 unforced errors.

Gauff did manage to save two more match points when serving at 5-2, but Halep won the next game to reach the quarterfinals.

Earlier, Serena Williams advanced to the Wimbledon quarterfinals for the 14th time, beating Carla Suarez Navarro 6-2, 6-2.

In the men’s draw, four-time champion Novak Djokovic advanced along with two-time winner Rafael Nadal. Djokovic beat Ugo Humbert 6-3, 6-2, 6-2 while Nadal defeated Joao Sousa 6-2, 6-2, 6-2.

Roger Federer, the record eight-time champion at the All England Club, was playing Matteo Berrettini later Monday.

Williams, a seven-time Wimbledon champion who missed about a year of play while she had a baby in 2017 but returned to the tennis tour in 2018, entered the tournament without having played since the third round of the French Open — skipping the grass-court warm-up tournaments.

“I definitely haven’t had enough (matches),” said Williams, who had been dealing with an injured left knee. “I have more matches this week than literally the past five months. So, yikes.”

Williams reached the quarterfinals at the Australian Open, then retired from a match at Indian Wells, withdrew from matches in Miami and Rome, and then played at the French Open.

“I know that I can play, and now that I’m feeling better physically I almost feel a relief more than anything,” Williams said. “Like, OK, finally I can play tennis.”

Against Suarez Navarro, Williams won six straight games from 3-2 in the first set and broke for a 5-2 lead in the second. She easily closed it out from there.

Williams, who lost to Angelique Kerber in last year’s Wimbledon final, will next face Alison Riske, an unseeded American who upset top-ranked Ash Barty.

Williams’ last loss was also against American opposition, Sofia Kenin in the third round at Roland Garros.

“Well the last time I faced a fellow American I lost, so I definitely want to do well this time,” Williams said. “And yeah, she’s great on the grass. She took out the No. 1 player in the world who just won a grass-court tournament. I watched that match, so I’ll be ready for her.”

Riske ended Barty’s 15-match winning streak, and her chances of winning a second straight Glam Slam title.

Riske beat this year’s French Open champion 3-6, 6-2, 6-3 and will play in the quarterfinals of a major tournament for the first time.

“I haven’t been starting out fantastic in all my matches, but I knew I had the confidence that if I could manage my service games I was going to get looks on her serve,” said Riske, whose best previous showing was reaching the third round at Wimbledon and the fourth round at the 2013 U.S. Open. “I had to play aggressive. I had to take it to Ash.”

Barty was playing her first tournament as the No. 1-ranked player, and she started off by winning points with her serve against Riske.

In the opening service game, the top-seeded Barty won all four points with aces. She won two more points in her next game with aces, as well. She finished the match with 12 of them.

But Riske took her chances when she got them, breaking Barty four times on four attempts, including to take a 5-3 lead in the deciding set before serving it out.

“I was sticking to how I wanted to play,” Barty said. “Then in the second set, I think my serve let me down. I let Alison get back into the match too many times, having looks at second serves.”

Also, No. 19 Johanna Konta eliminated two-time champion Petra Kvitova 4-6, 6-2, 6-4, No. 8 Elina Svitolina beat No. 24 Petra Martic 6-4, 6-2, Zhang Shuai defeated Dayana Yastremska 6-4, 1-6, 6-2, Karolina Muchova beat No. 3 Karolina Pliskova 4-6, 7-5, 13-11, and Barbora Strycova came from a set and break down to beat Elise Mertens 4-6, 7-5, 6-2.

Svitolina won six straight games from 4-4 in the first set. Martic called for medical treatment on her left leg after the first game of the second set and her movement appeared to be hampered the rest of the way. She asked for treatment again at 4-1.

In other men’s matches, No. 21 David Goffin beat Fernando Verdasco 7-6 (9), 2-6, 6-3, 6-4, Sam Querrey eliminated Tennys Sandgren 6-4, 6-7 (9), 7-6 (3), 7-6 (5), and Roberto Bautista Agut defeated Benoit Paire 6-3, 7-5, 6-2.