Russian sixth seed Dementieva showed incredible courage, battling against a strained thigh, stomach pains and her own serving demons to end second seed Amelie Mauresmo's dream of a first grand slam title with a 4-6, 6-4, 7-6 victory.
But the real fireworks came later on Tuesday on a crackling Arthur Ashe Court when eighth seed Capriati downed twice former champion Serena Williams 2-6, 6-4, 6-4 after two hours six minutes of unrelenting theatre.
That was followed by another awesome display of power by defending men's champion Andy Roddick as he blazed past Spain's Tommy Robredo 6-3, 6-2, 6-4 to advance to the last eight.
The 17th meeting between the old foes had everything the crowd could have asked for as two of the heavyweights of women's tennis went toe-to-toe under the Flushing Meadows lights.
Third seed Williams took the first set with two service breaks but the 28-year-old Capriati recovered her senses in time to set up a thrilling, and controversial, finale against her fellow American.
In the opening game of the third set, Williams served at deuce and hit a backhand winner inside the line which was correctly called good by the line judge, only for Portuguese umpire Mariana Alves to call "advantage Capriati".
Williams's disbelief turned to fury as she argued at the umpire's chair, but to no avail as Capriati broke serve. Williams broke back to level at 1-1, only to surrender serve again in the next game.
Capriati moved 5-4 ahead to serve for the match but was given another let-off at 0-15 when a Williams pass was again called out despite appearing to hit the line.
FOUGHT FEROCIOUSLY
To her credit Williams kept her cool and fought ferociously to stay in the quarter-final, saving two match points before a netted backhand ended the contest.
"She came out firing and I started tight but I just tried to stay with her," Capriati told reporters after clinching a place in the semi-finals for the fourth time.
"I just didn't want to let it slip away. It shows how good life is, you keep getting chance, after chance, after chance. I can't stop this now."
Williams she had received an apology from tournament officials, before adding: "She (the umpire) must have gone temporarily insane. I feel like I've been cheated. I'm very upset and very angry."
The crowd was still buzzing when Roddick walked out for his fourth-round match.
The American, who blitzed his way through the first three rounds without dropping a set, turned on the power once again to demolish 18th seed Robredo.
The second seed, looking to become the first man to win back-to-back titles since Australian Patrick Rafter in 1997 and 1998, bashed 36 winners.
Roddick now faces big-serving Swede Joachim Johansson after the 28th seed took his ace tally for the tournament to 76 in a 6-2, 6-3, 6-2 defeat of Frenchman Michael Llodra.
Earlier, Frenchwoman Mauresmo's grand slam jinx struck again as she missed the chance to overhaul Justine Henin-Hardenne at the top of the world rankings.
"I want to achieve a grand slam victory, I just hope it's going to happen very soon," said Mauresmo, who can still reach number one if American Lindsay Davenport fails to win the title.
OBVIOUS DISCOMFORT
Dementieva served 15 double faults and was in obvious discomfort from her stomach upset as she dragged herself back to win a contest containing 14 service breaks.
The Russian, who beat Mauresmo in the quarter-finals of the French Open, showed huge mental strength to stay in touch during the second set when she was twice down a break.
By the time the third set reached the tiebreak she was often bent double between points, but she summoned up her last reserves of energy to take it 7-1.
The early action on Arthur Ashe Court was tame by comparison as 2001 champion Lleyton Hewitt cruised through to the quarter-finals, beating unseeded Slovak Karol Beck 6-4, 6-2, 6-2.
Fourth seed Hewitt, still to drop a set in this year's tournament, broke in the ninth game of the first set and never looked back.
"He went away a bit after I broke," said Hewitt, who will play Tommy Haas for a place in the semi-finals.
"I'm very happy with where my game is at and am ready to take it up a notch," added the former world number one.
Haas, showing signs of the form that took him to two in the world before he was sidelined by shoulder surgery, saved three set points in the opening set against teenage Czech Tomas Berdych before progressing 7-6, 6-1, 7-5.
