Stephens beat Sevastova in the same round a year ago en route to her first grand slam title, but she missed numerous chances to grab an early lead in the rematch and could never get back into the match.
In her first grand slam semi-final, Sevastova, the No.19 seed from Latvia, will play Williams.
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.
"It's cruel, sometimes, tennis", Thiem said, calling the loss "the first really epic match I've played". "I still know that no matter whether I'm in the semifinals or the finals, I have a really long way to go".
If she wins it would be her 24th major title - equalling Australian Margaret Court's all-time record.
The departures of Stephens and Pliskova left Keys and Williams - seeded 17th despite her world 26 ranking - as the highest seeds remaining in the women's draw.
Williams has performed wonders already by reaching the Wimbledon final, less than 11 months after the birth of her daughter Alexis Olympia.
Pliskova, who managed to convert just two of 12 break point chances in the contest, said Williams was simply too good in the important moments.
"I feel like right now I'm playing free because I was having a baby this time previous year, so I have nothing to prove.
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". For Williams her service game wasn't so easy with Pliskova getting into her element ready to brake Williams again. He's into the semifinals for the second year in a row, where he lost to top-ranked Rafael Nadal last year.
Naomi Osaka of Japan made her first career Grand Slam final, beating 2017 U.S. Open runner-up Madison Keys 6-2, 6-4.
Williams put aside some early shakiness and an early deficit, turning things around with an eight-game run en route to a 6-4, 6-3 victory over No. 8 seed Karolina Pliskova for a spot in the semifinals.
"She got a little bit lucky, I think, on some breaks in the first set. I mean, there is still semi-finals", she said.
Pliskova is asked about the muggy conditions and says that she's grateful she hasn't played any three-setters in NY, having advanced to the quarters without dropping a set.
"Still long way to go, I think", the Latvian said.
"Against Serena or Karolina, it's going to be a tough match - they play aggressive, they serve big", Sevastova said.
It took Williams a while to get into it but it was inevitable that the American eventually would fight back, and in this game she did delivering punishing hard forehands which forced the errors out of Pliskova. Add that to her usual powerful baseline game and always superb serve, which reached 120 miles per hour (194 kph), and this really was no contest at all.
"I've seen her play a lot".