WIMBLEDON, England -- Serena Williams was pushed to three sets on Centre Court before beating fellow American Alison Riske 6-4, 4-6, 6-3 to reach her 12th Wimbledon semifinal.

Williams broke for a 5-3 lead in the third set when Riske double-faulted on the fourth break point of the game, and then served out the win with an ace. Williams said it makes the competitiveness of the field "super exciting."

"You want to come to see some tennis," she said. "I think, if anything, it speaks for the level of all players, the depth of the women's field. Everyone is playing incredibly tough."

Williams was down a break twice in the opening set but came back both times and broke again in the final game. But in the second set, Riske broke for a 5-4 lead before holding serve to even the match.

Riske converted all five break points she created, while Williams needed 16 opportunities to break six times.

"She was really so close to taking the win today," Williams said. "She was playing her heart out. She had nothing to lose and I realized I didn't either. I need to just do better."

Williams is seeking an eighth Wimbledon title and 24th Grand Slam victory, while Riske was playing her first quarterfinal at a major tournament. Williams was asked if she feels pressure as her career goes on.

"I definitely feel a lot more pressure as I got older. Now that I am past older, I feel a little bit less pressure," she said smiling. "I'm so serious. I definitely felt pressure, like not so much when I was younger, and then older, yes, yes, yes.

"Now it's like I've done everything I need to do. Like, I've had a great career, so I don't feel the pressure as much any more."

Williams was fined $10,000 for an incident on a practice court this week.

"I just threw my racquet," she said. "I got fined."

Williams was due back on Centre Court later Tuesday to play mixed doubles with Andy Murray.

Simona Halep reached the Wimbledon semifinals for the second time, beating Zhang Shuai 7-6 (4), 6-1.

Halep was broken in her first service game as Zhang raced to a 3-0 lead in the first set, and the Romanian faced four more break points at 4-1. But she eventually held serve in a game that featured six deuces, and then broke back to make it 4-3. In the tiebreaker, she converted her first set point with a forehand winner.

In the second set, Halep broke twice for a 5-1 lead and converted her second match point with a forehand at the net.

The 2018 French Open champion is looking for her second career Grand Slam title but had not reached the semifinals at the All England Club since 2014. She will next face either Elina Svitolina or Karolina Muchova.

"I just came here relaxed," Halep said. "I came here motivated to see how good I can be on grass. I'm happy on court. I think this helps me a lot."

Also Tuesday, Garbine Muguruza, the 2017 Wimbledon champion, has split with her coach of about four years, Sam Sumyk.

The champion at the All England Club in 2017 and French Open in 2016 wrote on Twitter that it was "the end of an Extraordinary ride. ... Beyond grateful for this last 4 years. Merci Sam."

Besides winning two major titles, Muguruza also rose to the No. 1 ranking in 2017. She is currently ranked 27th and lost in the first round at this year's Wimbledon tournament.

Information from The Associated Press was used in this report.