Serena Williams survived a test from fellow mother Tsvetana Pironkova to reach the U.S. Open semi-finals, while Daniil Medvedev defeated compatriot Andrey Rublev in straight sets to book his ticket into the tournament's final four on Wednesday. 

Williams, a six-times champion in New York, struggled with unseeded Pironkova of Bulgaria early but fought back from one set down to secure a 4-6 6-3 6-2 quarter-final victory. 

She kept alive her ongoing quest for a record-tying 24th Grand Slam singles title, refusing to let happen her earliest U.S. Open exit in 14 years. 

"I'm happy to be standing here... because at one point I was pretty close to not being here," Williams said during the on-court winner's interview. "I keep fighting and one thing that I am super excited about is I never give up."

Pironkova, back after an absence of more than three years during which she had a baby in early 2018, made a nice start as she consolidated a break to go ahead 4-2 in the first set.

After Williams saved two set points on serve to pull within 4-5, the Bulgarian held her nerve and secured the opener with a backhand cross-court winner.

In the second, Williams won a 24-point rally to break and open up a 5-3 lead and then, after falling behind 15-30 on her serve, fired three consecutive aces to force a decider.

Williams broke to start the third set in a game during which she even hit a left-handed shot, and never looked back as Pironkova suddenly started to run out of gas.

Despite the loss, Pironkova's performance won a lot of respect from her opponent across the net. 

"Whenever you can birth a baby honestly you can do anything and I think we saw that with Tsvetana today. She played unbelievable," said Williams. 

Williams said she was in awe of Pironkova's run to the last eight of the 2020 U.S. Open, which is Pironkova's first professional tournament since Wimbledon in 2017. 

"I couldn't even do that. I could barely win a match when I came back so she's incredible."

"It's like you play a match and you go home and you're still changing diapers. It's like a double life, it's really surreal," she added.

Up next for the 38-year-old Williams, who has been pushed to three sets in her last three victories, is former world number one Victoria Azarenka of Belarus, who defeated Elise Mertens 6-1 6-0 to storm into her first U.S. Open semifinal since 2013. 

Over to the men's singles, third seed Daniil Medvedev defeated 10th-seeded Russian compatriot, also his childhood friend Andrey Rublev in straight sets 7-6 (8/6), 6-3, 7-6 (7/5 )to reach the semi-finals of the U.S. Open on Wednesday.

Rublev raced to a 5-1 lead in the first-set tie-break before Medvedev, last year's US Open runner-up, hauled back to win it 8-6 with an ace.

"In these moments you feel like you have to fight for every point, but you're also thinking, 'Okay, I have probably lost this set,'" said Medvedev.

"I felt like I could get in trouble so I was really happy to get the winner in this tough tie-breaker."

In what was a baseline slugfest for much of the match, big-hitting Medvedev smashed 16 aces and won 89 percent of first-serve points.

His defensive game was also superior, making 75 per cent of returns to just 54 per cent by Rublev.

The 24-year-old will play either second seed Dominic Thiem or Australia's Alex de Minaur, seeded 21st, in the last four on Friday.

(With input from agencies)

(Cover image: Serena Williams of the United States reacts to breaking Tsvetana Pironkova of Bulgaria in the second set during her women's singles quarterfinal match on Day Ten of the 2020 U.S. Open at the USTA Billie Jean King National Tennis Center in New York, September 9, 2020. /VCG)