Sloane Stephens has reached her first grand slam semi-final since 2013 by beating 16th-seeded Anastasija Sevastova 6-3 3-6 7-6 (4) at the US Open.
The 83rd-ranked Stephens has won 13 of her past 15 matches.
She was one of four American women in the quarter-finals at Flushing Meadows, the most since five made it that far 15 years ago.
Sevastova beat Maria Sharapova in the fourth round and was trying to reach the first major semi-final of her career.
She received treatment from a trainer late in the first set at Arthur Ashe Stadium on Tuesday for a problem with her upper right leg.
Stephens went 11 months between tournaments because of surgery on her left foot in January, returning to competition at Wimbledon in July.
Her only previous trip to the final four at a major tournament came at the Australian Open four years ago.
Stephens will play either Venus Williams or Petra Kvitova, who face off at Arthur Ashe Stadium later on Tuesday.