Jessica Pegula scripted an astonishing comeback to defeat Karolina Muchova and reach her first Grand Slam final at the US Open on Thursday where she will take on big-hitting Aryna Sabalenka for the title.

World number six Pegula of the United States battled back from a set and a break down to record a gutsy 1-6, 6-4, 6-2 victory over her 52nd-ranked opponent from the Czech Republic.

Double Australian Open champion and world number two Sabalenka reached her second successive US Open final by seeing off another American, Emma Navarro 6-3, 7-6 (7/2).

"I thought I was lucky to still be in it," admitted Pegula.

"She made me look like a beginner, she was destroying me and I was about to burst into tears but it all came down to small moments. I don't know how I turned that around."

Pegula has now won 15 of 16 matches on the US summer hard court swing which saw a title in Toronto and defeat in the Cincinnati final to Sabalenka.

"It's a chance for revenge, but she'll be tough to beat," said Pegula.

Pegula, playing in her first Grand Slam semi-final after six painful quarter-final defeats, looked down and out at one stage on Thursday.

The 30-year-old lost the first set in just 28 minutes and was quickly 2-0 down in the second and had to save break point to avoid slipping 3-0 down.

That was the cue for many spectators to leave the Arthur Ashe Stadium not wanting to witness a second American defeat in quick succession.

However, Pegula dug deep to spark an incredible recovery during a rollercoaster conclusion to a thrilling semi-final as she raced away with eight of the last 10 games.

Muchova's 40th unforced error of the contest sealed her fate.

'Too late to cheer for me'

Earlier Thursday, Sabalenka went through to the final by seeing off Navarro as well as a partisan home crowd.

Navarro, playing in her first semi-final at the majors, went down fighting when she clawed her way back from 3-5 down in the second set to take her chances in a tiebreak.

Sabalenka sent down 34 winners to the 13 of the 23-year-old American.

"Guys, now you're cheering for me. Wow. It's a bit too late," Sabalenka told the Arthur Ashe Stadium crowd.

"It really means a lot and even though you guys were supporting her still I had the goose bumps for you cheering. It was an incredible atmosphere."

The Belarusian added: "I don't really care who I face in the final because today was a good test of the crowd cheering for my opponent. I don't care. I'm ready to face whoever."

Navarro, who had lost in the first round on her two previous appearances at the US Open, admitted that the occasion may have caught her on the hop.

"Maybe I didn't deal with it 100% the way I wanted to throughout the match, but it's a feeling that I'll definitely be seeking again," she said.

Sabalenka, runner-up to Coco Gauff of the US in New York last year, called on her raw power to secure the first set, nullifying Navarro's smart use of the angles.

Although an early break was cancelled out by the American, the 26-year-old edged ahead again in the sixth game before securing the opener on the back of 16 winners, twice as many as Navarro.

Sabalenka stretched to a 3-2 lead with a break in the second set with Navarro having to fend off more break points to avoid slipping to a 2-5 deficit.

That breathed new life into her challenge and she broke Sabalenka when her rival served for the match, pulling level for 5-5.

She even enjoyed a mini-break in the tiebreak before Sabalenka stormed back for victory.