WIMBLEDON, England -- Ash Barty's winning streak is over, and so are her chances of winning a second straight Grand Slam title.
The Australian had won 15 matches in a row, including the French Open title, but she lost to Alison Riske 3-6, 6-2, 6-3 at Wimbledon on Monday, giving the unseeded American a spot in the quarterfinals of a major tournament for the first time.
Riske will next face Serena Williams on Tuesday in the quarterfinals. Williams defeated Carla Suarez Navarro 6-2, 6-2 on Monday.
"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 in a major was reaching the third round at Wimbledon and the fourth round at the 2013 US 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 won't lose that ranking despite the loss. Karolina Pliskova could have supplanted Barty at the top of the rankings if she had reached the Wimbledon women's final, but she was upset on Monday.
Pliskova, seeded third at Wimbledon, fell to Karolina Muchova, who narrowly avoided a deciding-set tiebreaker, prevailing 4-6, 7-5, 13-11 to reach the quarterfinals.
Barty 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 and finished the match with 12.
But Riske took her chances when she got them, breaking Barty four times on four attempts, including a break to take a 5-3 lead in the deciding set before serving it out.
"Tough one. I think I started well. 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.
"Overall, I didn't play a poor match. When I needed to, when the big moments were there, Alison played better today. Tough one to swallow but I lost to a better player."
Riske has never faced Williams in a singles match. They were doubles partners in the 2015 Fed Cup when the U.S. faced Italy. Riske said Monday she's "ready for a war."
"She's the greatest athlete I think that's ever been on the women's side. It's going to be a huge challenge, but I'm really looking forward to it," she said.
Williams, who missed about a year of play when she had a baby in 2017 but returned to the tour in 2018, had not played before Wimbledon 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, 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, gets Riske next.
Williams' last loss came against American opposition, to 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."
Muchova won the last three games of her match against Pliskova and converted her third match point with a return that clipped the net cord.
Pliskova broke for an 11-10 lead but couldn't serve out the match as she was broken back at love.
If the score had reached 12-12, the pair would have played the first deciding-set tiebreaker since Wimbledon introduced new rules for this year's tournament.
In other women's matches Monday, No. 8 Elina Svitolina beat No. 24 Petra Martic 6-4, 6-2, and Zhang Shuai defeated Dayana Yastremska 6-4, 1-6, 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.
Barbora Strycova came from a set and break down to beat Elise Mertens 4-6, 7-5, 6-2. Mertens led 5-2 in the second set but lost nine straight games to go 4-0 down in the second.
Strycova next faces Johanna Konta who reached her second Wimbledon quarterfinal by coming from a set down to knock out two-time champion Petra Kvitova.
Konta, the only British player left in the tournament, won 4-6, 6-2, 6-4 on Centre Court. Konta missed two match points at 5-2 in the decider but served it out on her next opportunity when Kvitova sent a shot long.
Kvitova hit 40 winners to Konta's 22, but also had 34 unforced errors compared to 21.
In 2017, Konta became the first British woman since Virginia Wade in 1978 to reach the semifinals at the All England Club. She lost in the second round last year.
Venus Williams, meanwhile, lost for the second time at this year's Wimbledon tournament, eliminated from mixed doubles in the second round.
Williams, a five-time champion at the All England Club, lost to Coco Gauff in the first round of the singles tournament last week. In mixed doubles, Williams and partner Frances Tiafoe lost to 12th-seeded Franko Skugor and Raluca Olaru 6-3, 6-1.
The Associated Press contributed to this report.