Sevastova, who avenged last year's quarter-final loss to Stephens in NY to reach her first Grand Slam semi, converted five of her eight break points and saved seven of the nine against her on the Arthur Ashe Stadium court.
Their last meeting was at Indian Wells previous year, when the American won 6-1, 6-4.
Serena Williams of the United States returns the ball during the women's singles quarter-final match against Karolina Pliskova of Czech Republic on Day Nine of the 2018 US Open at the USTA Billie Jean King National Tennis Center on September 4, 2018 in the Flushing neighborhood of the Queens borough of New York City.
While she gave up two straight games to Pliskova, and then narrowly dropped a third later on, Williams quickly finished her off to take the second set 6-3 and advance to the semis.
Keys has beaten Osaka all three times they have played, including this year at the French Open and two years ago at the U.S. Open when she rallied from a 5-1 deficit in the third set.
"I just wanted to play better", Williams said of her mindset as she trailed 3-1 in the first.
If she wins it would be her 24th major title - equalling Australian Margaret Court's all-time record.
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World number three Stephens, refused to blame the punishing afternoon heat and humidity for her lapses against Sevastova, including an inability to convert any of seven break chances in the opening set.
This time previous year, Serena Williams was battling through a very different challenge as she gave birth to her first child, Alexis Olympia. "I really felt like I was playing well in Cincinnati, even though I lost.
I feel like she has a big chance because the players which stayed [in the tournament], I feel like she can beat all of them". A piercing backhand off Pliskova's serve set up advantage point for point for Williams but the Czech produced a cracking serve to level it back at deuce.
Williams fell behind early, and looked visibly frustrated in the first few games after several errors.
Serena Williams was a bit shaky at the start of her U.S. Open semifinal.
The world number 8 Karolina Pliskova led the first set against Williams by 3-1.
"So I thought I gotta try harder".
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The win came in her 100th match on Arthur Ashe Stadium, where she won the first of her six US Open titles in 1999.
"She's been playing really well for actually a long time", Williams said of her semi-final opponent. You can't control the weather, can't control what the tournament is going to do.
"So obviously I know her game really well".
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She broke again to go up 2-0 in the second set before Stephens, pleading with herself to fight, broke back.
The most noticeable improvement in Pliskova's game since her 2016 semifinal shock upset over Williams is her movement around the court.
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