US Open: Serena Williams 'fights hard' to reach semi-finals, Dominic Thiem to face Daniil Medvedev

American 23-time Grand Slam champion to face former world No 1 Azarenka for a place in the final

Serena Williams celebrates after winning a point against Tsvetana Pironkova during the US Open quarter-finals. Reuters

Serena Williams waves to the Jumbotron as she walks off court after winning her US Open quarter-final. AFP

Serena Williams during her US Open quarter-final match against Tsvetana Pironkova. PA

Serena Williams returns the ball during her US Open quarter-final match against Tsvetana Pironkova. AFP

Victoria Azarenka hits a backhand against Elise Mertens during the US Open quarter-finals. Reuters

Victoria Azarenka gives a thumb up after defeating Elise Mertens in the US Open quarter-finals. AP Photo

Victoria Azarenka hits a backhand against Elise Mertens during the US Open quarter-finals. Reuters

Victoria Azarenka prepares to hit a forehand to Elise Mertens during the US Open quarter-finals. AFP

Daniil Medvedev greets Andrey Rublev at the net after their US Open quarter-final match. Reuters

Daniil Medvedev celebrates after winning his US Open quarter-final against Andrey Rublev. AFP

Daniil Medvedev hits a serve to Andrey Rublev durring the US Open quarter-finals. AFP

Daniil Medvedev returns the ball to Andrey Rublev during the US Open quarter-finals. AFP

Dominic Thiem celebrates after defeating Alex de Minaur in the US Open quarter-finals. EPA

Dominic Thiem taps racquets with Alex de Minaur after their US Open quarter-final match. EPA

Dominic Thiem returns a volley during his US Open quarter-final match against Alex de Minaur. AFP

Dominic Thiem returns to Alex de Minaur during the US Open quarter-finals. AP Photo

Serena Williams said it was her "never give up" attitude that saw her battle past Tsvetana Pironkova to reach the US Open semi-finals where she will face long-time rival Victoria Azarenka.

Williams, seeded third, came from a set down to overcome unseeded Bulgarian Pironkova 4-6, 6-3, 6-2 and keep alive her bid for a 24th singles Grand Slam title at Flushing Meadows.

Later, Azarenka blew away Belgium's Elise Mertens 6-1, 6-0 as she continues her hunt for a first Grand Slam title in seven years.

Williams' comeback included two astonishing shots with her left hand that contributed to winning points.

"At that point I was fighting so hard. I was just trying to do everything I can whether righty or lefty," she said.

A sluggish start saw the American lose the first set, but she fought back to take the next two in two hours and 12 minutes inside Arthur Ashe Stadium.

"I never give up and you know I just gotta keep going," Williams said.

Williams smashed 20 aces, the most she has had in eight years, taking her tally for the tournament to 64 so far. The record for the most number of aces in a US Open in the women's singles is 70, set by Williams in 1999 when she won for the first time.

"Sometimes when I'm serving, I just tell myself, I don't care if my arm falls off, I'm going to keep serving," she said.

Williams is looking to match Margaret Court's record for the most number of singles Grand Slam wins.

It has been more than three years since she won her 23rd at the 2017 Australian Open - when she was already pregnant with daughter Olympia.

Williams has come close since, reaching four major finals only to come away empty handed.

In the last four she will renew her rivalry with former world No 1 Azarenka who demolished Mertens in just one hour and 13 minutes. Williams defeated Azarenka in the final of the US Open in 2012 and 2013.

The last few years of Azarenka's career have been interrupted by a custody battle over her son who was born in 2016.

Azarenka said she found it "hard to accept losses" when she was young and at the top of her game, but was now benefiting from a more philosophical attitude.

"I enjoy the moment no matter what it is. I take life as it comes," Azarenka said. "It took me a long road to come here with a lot of struggles, a lot of understanding, forcing me to find this route, this path, if you can say that. But I'm here and I'm happy."

In the other women's semi-final, 2018 champion Naomi Osaka will face American Jennifer Brady after both players came through their respective quarter-finals on Tuesday.

Thiem faces Medvedev in men's semis

In the men's draw, Russia's third seed Medvedev downed compatriot and childhood friend Andrey Rublev 7-6, 6-3, 7-6.

Rublev, 22, 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.

Medvedev, 24, smashed 16 aces and won 89 per cent of first-serve points. He made 75 percent of returns to Rublev's 54 per cent.

"One point decided two sets so it was a really tough match and Andrey was playing unbelievable actually," Medvedev said.

Medvedev will play second seed Thiem who demolished Australia's Alex de Minaur, seeded 21st, 6-1, 6-2, 6-4.

"I had a great feeling from the first moment on," said the Austrian.

Fifth seed Alexander Zverev of Germany faces Spain's 20th seed Pablo Carreno Busta in the competition's other semi-final.

The disqualification of Novak Djokovic for accidentally hitting a line official with a ball during the last 16 means the US Open will crown a first-time male Grand Slam winner on Sunday.