Jessica Pegula was confident she could defeat a "prime" Iga Swiatek to reach her first grand slam semi-final at the US Open. (Full Coverage | More Tennis News)
Pegula, who had lost her previous six slam quarter-finals, came through in straight sets at Flushing Meadows to stun the world number one.
She also became the fourth American in the Open Era to reach the women’s singles semi-final at the Canadian, Cincinnati and US Open in a calendar year after Rosemary Casals (1970), Serena Williams (2013-15) and Sloane Stephens (2017).
Pegula's triumph set up a meeting with Karolina Muchova for a place in the final after the Czech overcame illness to beat Beatriz Haddad Maia, also in straight sets.
With Emma Navarro along with Frances Tiafoe and Taylor Fritz advancing to the final four, it is the first time two Americans have reached the semi-finals in the women’s and men’s singles at the US Open since 2003, with Jennifer Caprati, Lindsay Davenport, Andy Roddick and Andre Agassi the first do so.
It also marked Pegula's fourth victory over the Pole, with the latest win considered one of her best.
"I've been [to the quarter-finals] so many freaking times but I kept losing," Pegula said.
"Finally - finally - I can say I'm a semi-finalist. Thank you to the crowd, you carried me through that last game.
"To do it at prime time against the number one player in the world - it's crazy, but I knew I could do it."
