The 30-year-old Pierce, who was born in Montreal to a French mother and an American father, will play either Maria Sharapova, of Russia, the top seed, or the Belgian Kim Clijsters, in the final here tonight.
Pierce, the 12th seed, defeated the sixth-seeded Elena Dementieva, of Russia, the runner-up last year, 3-6, 6-2, 6-2, and has now won all six semi-finals she has played.
But the talking point yesterday was not Pierce's progress but her 12-minute injury time-out at the end of the first set, which was within the rules but threw her opponent off her game.
"You can change the game by winning incredible points or by taking a 12-minute time-out, which is in the rules but which I think was not fair play," said Dementieva. "If that's the only way she can beat me, that's up to her. You have to be patient when you play Mary. You have to be ready for that sort of thing."
Pierce was treated for consecutive injuries: a lower-back problem and a strained right thigh, a legacy of her quarter-final against Amélie Mauresmo.
While the delay eased Pierce's pain, it did nothing for her opponent's concentration. Dementieva, who dominated the first set after Pierce double-faulted twice to lose the opening game. Dementieva broke again in the ninth game.
The Russian was moving smoothly, playing her groundstrokes with impressive assurance, and serving particularly well considering she had double-faulted 62 times in her five matches.
Although Dementieva held to love in the opening game of the second set, she was broken in the third game, hitting a backhand long to be broken for 2-1. Pierce, settling into her game, broke again for 5-2, after Dementieva double-faulted for only the second time (she finished the match with six).
The final set opened with Dementieva saving a break point, a prelude to more problems for the Russian, who tried to finesse her back into the match, and failed. Pierce, moving like a thoroughbred, broke in the third and fifth games and saved three break points before serving out after two hours and three minutes.
The big question in the closing stages of the men's singles is who is going to stop Roger Federer from winning his second consecutive US Open and fifth Grand Slam title, and in the process passing Pete Sampras's record of 34 wins in a row on concrete courts?
Lleyton Hewitt, who is due to play the Swiss world No 1 in the men's singles semi-finals today, has lost his last eight matches in a row against the Wimbledon champion, including a drubbing, 6-0, 7-6, 6-0, in last year's final at Flushing Meadows.
The two Americans, ancient and modern, Andre Agassi and Robby Ginepri, who meet in the other semi-final, have also been at a loss to fathom a way past Federer. He has won his last seven matches against the 35-year-old Agassi, and both contests against the 22-year-old Ginepri.
David Nalbandian, of Argentina, used to have Federer's measure when they were juniors and won their first five matches after they graduated to the ATP Tour.
Federer, having learned from those defeats and developed an awe-inspiring all-court game, defeated Nalbandian for the third time in a row when they met in the quarter-finals on Thursday night.
Although Nalbandian broke in the opening game and led, 4-2, in the second set, Federer won with comparative ease, 6-2, 6-4, 6-1, after an hour and 40 minutes. Nalbandian became so frustrated by Federer's mastery that he decapitated his racket on his courtside chair.
The match contrasted sharply to the 20 five-setters recorded in the men's draw, including Hewitt's quarter-final against Jarkko Nieminen, of Finland, Ginepri's conquest of Guillermo Coria, of Argentina, and Agassi's marathon against the Belgian Xavier Malisse followed by his epic against James Blake.Reuse content