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.
