Sevastova got her revenge from losing last year's US Open quarterfinal with Stephens.
This year, Williams' French Open appearance - her first Grand Slam since the birth of her child, was watched closely by fans, and the American progressed to Round 4 of the tournament having dropped only a set.
Asked about Pliskova's game in particular, the six-time US Open champion is brief and to the point: "She just does everything well".
Interestingly, Williams' quarter-final win on Wednesday marked her 100th match at the Arthur Ashe Stadium - the venue of Williams' first-ever Grand Slam singles title 19 years ago, in 1999 at the age of 18. It would be Williams' first major since she won the 2017 Australian Open. "I felt so bad because everyone here was cheering, and I wasn't winning" Serena said.
"I still know that no matter whether I'm in the semi-finals or the finals, I have a really long way to go to win that".
Williams has done her part, making her way to the last four with her usual efficiency while throwing in an occasional wobble just to keep things interesting but not scary.
Williams looked destined to drop the first set against eighth seed Karoline Pliskova, trailing 4-2 and down a break.More news: Taliban says founder of militant Afghan Haqqani network dies
Indeed Stephens fought back to narrow the gap to 4-3, but Sevastova grabbed another break with a well-timed drop shot for a 5-3 lead and sealed the victory on her third match point when a tired Sephens put a backhand into the net.
"I really feel like right now I'm playing free because I was having a baby this time previous year, so I have nothing to prove".
The 36-year-old, though, quickly found her groove.
The US superstar, chasing a record-equalling 24th major title, surrendered an early break to Pliskova, but she roared back with a streak of eight straight games to put away the opening set and take a 4-0 lead in the second against the woman who beat her in the semi-finals at Flushing Meadows in 2016.
Sevastova first shot to fame in 2010 when she reached the semifinal of Monterrey Open, beating the then-world No. 9, Jelena Jankovic.
"Still long way to go, I think", the Latvian said. Having achieved as much as Williams might make anyone complacent and a fan may not even have blamed her for hanging up her racquet - after all, she has more than proved her status as the Greatest of All Time. That was the blueprint Williams followed Tuesday to fight her way back from down 3-1 in the first set and save three break points to take the game and keep fighting. All of those matches take place on the outer, roofless courts at Billie Jean King National Tennis Center where there is no shade, but temperatures soared on the tournament's main stage as well. "I think that's what happened today".
"I think you need some time to look at the journey - it was an unbelievable journey these three to four years", she said.
Osaka is a fearless competitor who is capable of hitting through anyone on her day, but the Keys serve may just be the difference in this one. They've both played commanding tennis from the baseline this year, relying on similar tools. You can't control the weather, can't control what the tournament is going to do.