Victoria Azarenka stunned Serena Williams 1-6 6-3 6-3 in the semi-finals of the US Open, denying the American the chance of winning a record-equalling 24th grand slam singles title on home soil this year.

Azarenka will play Japan's Naomi Osaka on Saturday in her third final at Flushing Meadows, having lost the previous two to Williams in 2012 and 2013.

Azarenka said her mental game had been crucial in beating the 38-year-old Williams, who needed a medical timeout to have her left ankle retaped in the third set due to achilles trouble.

"I have been working on finding a calm mind and keep the body going," the Belarusian said in an on-court interview. "She dug me in a big hole in the first set. I needed to find energy to climb out of the hole and it wasn't easy."

Williams, who has been stuck on 23 Grand Slam titles since winning the 2017 Australian Open, said the defeat was "obviously disappointing".

"At the same time ... I did what I could today," she added.

"I feel like other times I've been close and I could have done better. Today I felt like I gave a lot."

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Six-times US Open champion Williams came out firing winners from all sides of the Arthur Ashe Stadium court while Azarenka struggled with her serve, committing four double faults in the first set and winning just six of 15 first-serve points.

She also had 10 unforced errors and appeared increasingly frustrated, with a mic catching her muttering a pair of expletives at herself before Williams took a 4-1 lead.

Williams wrapped up the opening set in little more than half an hour but Azarenka seized momentum in the second frame, breaking the third seed's serve with a pair of backhand winners from the baseline to take a 3-2 lead.

Playing her first grand slam semi-final in seven years, Azarenka got the better of Williams in some explosive exchanges of power-hitting while her serve also saw major improvement.

The 31-year-old took the set to force a decider after winning nearly 75% of her first-serve points and committing just one unforced error.

The twice Australian Open champion kept up the pressure in the third set and looked to be in a deep meditative state as Williams had her ankle taped, not even reacting as an insect landed on the bridge of her nose as she waited to resume the match.

Azarenka then sealed her place in the final with an ace. 

Earlier, Osaka saw off Jennifer Brady to march back into the final.

The 2018 champion, who was watched by her rapper boyfriend Cordae, has hit top form again at Flushing Meadows and came out on top of a high-quality match to triumph 7-6 (7-1) 3-6 6-3.

Brady has been the breakout star of the tournament and her performance here showed that America has yet another woman who can challenge for grand slam titles.

But that honour here will go to Osaka, who will take on Azarenka in her third major final.

The Japanese fourth seed came onto court wearing a mask bearing the name of Philando Castile, a 32-year-old black man who was shot dead by a police officer in 2016.

Playing for a cause bigger than herself has given Osaka a clear mindset on court, and she played a virtually immaculate opening set.

Brady, 25, had not dropped more than four games in a set all tournament, and certainly did not let the occasion get to her, the pair trading ferocious blows.

The American had the only chance to break in the seventh game but missed a return, and that was her achilles heel in the tie-break as Osaka marched to within sight of the final.

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But Brady fully believes she belongs with the best players in the world now, as she pushed ahead with an even better second set.

She came out on top of a pulsating rally to break for 5-3 and confidently served out to level the match.

The match was being played under the Arthur Ashe roof, with storms forecast in New York, and, as the rain hammered down, Osaka forged ahead in the decider.

Brady had not faced a break point for more than two sets but a loose game at 1-2 gave Osaka an opening and she pounced.

The net cord, which had favoured the American until that point, had a hand in it, while Brady declined to challenge a backhand called out on break point that was shown by HawkEye to be in.

That proved to be the beginning of the end of Brady's terrific tournament as Osaka held her nerve to serve it out.