Collins had already seen off Venus Williams in the quarter-finals but met her match against the Latvian, who was ruthless in the first set tie-break breaking her four times as Ostapenko took the first set.
Collins, belying her 93rd ranking, pounded victor after winner to beat fellow American Williams at her own game and set up a semi-final against French Open champion Jelena Ostapenko.
The US Open champion punched her ticket for today's showpiece with a 3-6, 6-2, 6-1 semi-final win over former world No. 1 Victoria Azarenka of Belarus on Thursday.
In a men's quarterfinal, 16th-seeded Pablo Carreno Busta wasted two match points on his serve in the second set before recovering - almost two hours later - and ousting No. 6 Kevin Anderson 6-4, 5-7, 7-6 (6) in a rematch of a U.S. Open semifinal from past year.
It's just I was fresher in the first set and getting to the ball at the right time.
Stephens beat three-time champion Victoria Azarenka 3-6 6-2 6-1 earlier in the day.
"After losing the first set, I was just like, Okay, I didn't really do anything wrong".
"I wound up dropping a great deal of matches in a row and I only attempted to keep the momentum moving".
"I honestly didn't feel good at all the whole match", admitted Azarenka afterwards.
Collusion craze: Mueller memo says Trump aide contacted 'assessed' Russian spy
Van der Zwaan's memo Tuesday describes the work he did at the law firm Skadden Arps, which eventually led to his guilty plea. The report defied the view held by the USA and the European Union that the case against her was politically motivated.
"I mean, she played very well and aggressively, and she went for every shot and it landed", explained Williams.
"Everything that is special or important to me I keep at my grandparents' house", she said, a big, toothy smile emerging from beneath her baseball cap.
The reigning French Open champion and a surprise American qualifier completed the Miami Open semifinals on Wednesday.
Stephens endured a torrid five-month losing streak following her shock triumph at Flushing Meadows a year ago, but since she snapped the run in Acapulco last month she has rediscovered her best tennis.
"Of course I was working on my mentality more, because I need my confidence and to be more consistent", she said.
Fourteenth seeded John Isner dominated with his serve in dismissing 19 seeded Hyeon Chung 6-1, 6-4 to make the elite eight in Miami.
Going into the match against Stephens, Ostapenko said, "I think I'm calmer and also mentally stronger than I have been".
"It was unlucky for me", Williams said.