Victoria Azarenka came from behind to stun Serena Williams in a three-set thriller and reach the US Open final on Thursday. 

The unseeded Belarusian fought back to overcome Williams, seeded third, 1-6, 6-3, 6-3 in 1hr 56min under the roof of Arthur Ashe Stadium.

Azarenka will now play Naomi Osaka of Japan in Saturday's final.

It will be the former world number one's first appearance in the final of a tennis major since Williams beat her in the final of the US Open in 2013.

"That's my favorite number so I guess that's meant to be," Azarenka said of her seven-year wait to return to a Grand Slam singles final.

"I'm very grateful for this opportunity. I'm so grateful to be able to play such a champion in the semi-finals," she added.

Two-time Grand Slam winner Azarenka had never beaten Williams before in a Grand Slam and she seemed to be heading for another agonizing loss after being hammered 6-1 in the first set.

But the 31-year-old turned the tide in set two, upping her service game and drastically reducing her unforced errors as she began to dictate lengthy rallies.

"I knew it's never over until I have another chance," said Azarenka.

The player from Belarus also suffered a heartbreaking loss to Williams in the 2012 US Open, when she served for the match at 5-3 in the third set only to lose 7-5.

Azarenka was determined that history would not repeat itself. Serving for the match, she hit a double fault before smashing her fastest serve of the night at 109 mph.

When Williams then challenged and lost an ace that was called in on the next point, it was game, set and match to Azarenka.

"I've been there before. Today was gonna be different," Azarenka said.

Her last Slam title came at the 2013 Australian Open, when she retained the crown she had won a year earlier.

The 31-year-old is enjoying a fairytale run at the US Open after claiming last month's Western and Southern Open tune-up event in New York. 

Azarenka became a mother in December 2016. A custody battle over her son has disrupted her career in recent years, but she is currently experiencing a renaissance and is up to 27th in the world rankings.

Williams had been hoping to match Margaret Court's record of 24 Grand Slam singles titles.

Williams plans to dust herself down after the disappointment of another failed attempt at a record-equalling 24th Grand Slam singles title and continue her chase at Roland Garros this month.

Williams crashed out of the US Open semi-finals on Thursday, blowing a set lead to lose in three hard-fought sets to former world number Victoria Azarenka.

"It's obviously disappointing but at the same time I did what I could today," a weary-looking Williams told reporters.

"It feel like other times I've been close and I could have done better and today I felt like I gave a lot," she added.

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

The 38-year-old has come close since, reaching four major finals only to come away empty handed, and needs one more to match Margaret Court's record of 24.

Many commentators suggested that this week at Flushing Meadows presented her with her best opportunity due to a depleted field caused by the coronavirus pandemic.

A bevy of top players, including world number one Ashleigh Barty, world number two Simona Halep and Canadian Bianca Andreescu -- who stunned Williams in last year's final -- skipped the event because of coronavirus concerns or injury.

A tournament victory for Williams would also have seen her break away from Chris Evert, on six titles, as the most decorated US Open champion of the women's game in the modern era.

But Williams fell at the penultimate hurdle to a rejuvenated Azarenka who bounced back from a 6-1 defeat in the first set to take the next two 6-3, 6-3.

It marked the first time that Williams had lost to Azarenka in almost a dozen Grand Slam matches.

Williams' presence at the rescheduled French Open, which starts on September 21, had been in doubt after she said earlier this week that she was undecided about whether to play.

Williams, who has a history of serious lung problems, said she wanted to talk to organizers about safety protocols after they said that some spectators would be in attendance.

After the French Open tournament director announced that all players will be housed in two designated hotels "without exception," Williams added that she had hoped she would be able to stay in her Paris apartment.

When asked Thursday night whether she would compete for a third title at Roland Garros, she answered: "Yeah, I mean, I'm definitely going to be going to Paris."

Meanwhile, fourth seed Naomi Osaka defeated 28th seed Jennifer Brady in a high quality three-setter to reach the US Open final on Thursday, crediting her run to a carefree attitude post-lockdown. 

The Japanese 2018 US Open champion overcame the 25-year-old American 7-6 (7/1), 3-6, 6-3 in 2hr 8min under the roof of Arthur Ashe Stadium.

Osaka, 22, will play either Serena Williams or Victoria Azarenka in Saturday's final. 

"I just felt like I was sticking it out, it felt like we were trading serves," Osaka said of the semi-final tussle.

"I think I tried to adjust a little bit on her serve in the third set so maybe that helped," she added.

Brady, 25, had not dropped a set during her run to her first Grand Slam semi-final. 

And the world number 41 went toe-to-toe with Osaka until the first set tie-break. 

The players traded six held service games each before Osaka the world number nine took control of the tie-break. 

At 2-1, Osaka won five points in a row, which included two unforced errors by Brady.

In set two the first seven games were held before Brady broke Osaka's serve to take a 5-3 lead. 

She then successfully held to take the match to a deciding third set as the duo traded big serves with powerful forehand groundstrokes.

The breakthrough in the deciding set came game when Osaka broke Brady's serve in a fourth game that included a lucky net cord point for the Japanese.

Osaka then held her own to go 4-1 in front with the help of another net cord point.

Brady refused to go quietly though, forcing Osaka to hold twice to ensure the win and a second US Open final in three years.

"I kind of consider New York my second home. I really love the atmosphere," Osaka said.

"Even though sadly there's no people here, I really feel like this court really suits me well," she added.

Osaka has now won her last ten matches.

"I just felt like I wanted to come out of quarantine being positive and not really caring if I win or lose," she said.

Osaka, of Haitian and Japanese heritage, walked onto the court wearing a facemask emblazoned with the name of Philando Castile, a black man who was shot dead by a police officer during a traffic stop in Minnesota in 2016. 

The 2019 Australian Open champion is wearing different masks honoring victims of racial injustice and police brutality throughout the tournament. 

She has already donned face coverings bearing the names of Breonna Taylor, Elijah McClain, Ahmaud Arbery, Trayvon Martin and George Floyd.