Unseeded Anastasia Myskina followed up her defeat of Justine Henin by reeling off 11 straight games to upset Martina Hingis in the Pilot Pen event.
Myskina won 6-7 (2-7) 6-4 6-0 in the quarter-finals of the New Haven tournament on Thursday night.
The Russian now has a semi-final against second seed Lindsay Davenport, who reached the last four with Venus Williams and Daniela Hantuchova.
Myskina found herself facing defeat in the second set, down 4-1 and a break point that would have left Hingis serving for the match.
But she stunned the former world number one by winning the final 11 games, finishing with five aces in her closing two service games.
Playing only her second tournament since ankle surgery in May, Hingis admitted she was struggling for fitness.
"Of course fatigue was a factor," she said. "Until a set and 4-1 I felt okay, but I was already trying to finish it off."
But she added: "I can be happy with the way I played.
"I lasted a set and a half and played really well until then, compared to last week."
In the other matches, Williams and Davenport took different routes into the semi-finals.
Top-seed and three-time defending champion Williams breezed by American qualifier Laura Granville 6-2 6-1 in under an hour.
But second-seeded Davenport had to slug it out for 90 minutes before beating Amelie Mauresmo 7-6 (7-5) 6-3.
Williams actually dropped serve in the opening game against the little-known Granville.
But she broke back immediately and then dominated with a display of fearsome serving.
Hantuchova advanced with a 6-4 6-3 win over Patty Schnyder of Switzerland.
And she is looking forward to a re-match with Williams, who needed three sets to beat her at this year's Australian Open.
"If I play like I did today I think I've got a good chance," the seventh seed said.
Thursday's results
Quarter-finals
(1) Venus Williams (USA) bt Laura Granville (USA) 6-2 6-1
(2) Lindsay Davenport (USA) bt (6) Amelie Mauresmo (Fra) 7-6 (9-7) 6-3
(7) Daniela Hantuchova (Slo) bt Patty Schnyder (Swi) 6-4 6-3
A Myskina (Rus) bt M Hingis (Swi) 6-7 (2-7) 6-4 6-0