Seven-time champion Serena Williams is just two wins away from equaling Margaret Court's Grand Slam titles record of 24 but she wobbled several times in her Wimbledon quarter-final against Alison Riske on Tuesday.

The 37-year-old, who recorded her 97th success at Wimbledon to move second on the women's all-time list, came under enormous pressure from her fellow American before prevailing in an engrossing contest, 6-4, 4-6, 6-3 in just over two hours.

Standing in Williams's way in what is her 12th Wimbledon semi-final will be unseeded Czech veteran Barbora Strycova.

The American says she will not be taking the 33-year-old lightly as she has ousted four seeds on her run to the last four.

"She's good on the grass," said Williams of Strycova.

"She knows what to do. She has a good all-around game.

"She's incredibly tricky. It's definitely not easy. But it's something I'm definitely geared up for."

It says a lot about the unpredictability of the women's game that Williams was the only player in the last eight who reached the same stage last year.

"It was really satisfying," said Williams, whose clay court campaign was affected by a knee injury.

"I wouldn't have won that match a couple of weeks ago."

Svitolina reaches first Grand Slam semi-final

Ukraine's eighth seeded Elina Svitolina came back from a break down in each set to beat Karolina Muchova 7-5, 6-4.

It will be the 24-year-old Ukrainian's first Grand Slam semi-final.

For Svitolina breaking new ground in terms of her career is exciting enough.

"It feels amazing," she said. "It is the first time in the semi-finals for me and I didn't expect it would happen here."

Svitolina enters her first career grass court meeting with No.7 seed Simona Halep, who holds off China's Zhang Shuai to reach semi-final.

Chinese-Belgian pair exit

Zheng Saisai from China and Joran Vliegen from Belgium failed to reach 4th round in mixed doubles at Wimbledon on Tuesday, after beaten by Eden Silva and Evan Hoyt from Britain.

Evan Hoyt and Eden Silva, the British mixed-doubles pair, came from behind to progress to the Wimbledon quarter-finals in 7-5, 6-7, 4-6.

Before that, Zheng and Vliegen defeated second-seeded Jean-Julien Rojer and Demi Schuurs from the Netherlands 6-4, 7-5 in 2nd round while Hoyt and Silva had beaten 16th-seeds Divij Sharan and China's Duan Yingying.

"Tennis in China is in a period of rising. I have prepared for the next hard court season," Zheng said.

(With input from AFP, Xinhua)