"It wasn't an easy match at all", said Williams, who last won the US Open title in 2014. But at 2-4, Williams found that extra gear that's only available in the deluxe champion package, and rattled off the next eight games, powering her way to the first set and a 4-0 lead in the second before the Czech ever won another game.
She will play 19th seed Anastasija Sevastova, who had earlier beaten the defending champion Sloane Stephens for a place in the final.
Serena Williams says she still has a "really long way to go" to win a record-equalling 24th Grand Slam singles title at the US Open.
Serving for the match at 5-3 Williams didn't allow any room for doubt, holding at love with two aces and an overhead smash followed by one last ace.
Before this tournament began, Williams downplayed her chances of taking the title here, citing the fact that this was just the third major she'd played since becoming a first-time mom previous year at this time. Williams donned an all-black catsuit to the tournament, and said it made her feel "powerful, like a superhero when I wear it, like a Queen from Wakand" - and perhaps nobody deserves the title of superhero, or Queen, on the tennis court more than Williams. "I kind of go out there and I just do the best that I can".
With Halep, number two seed Caroline Wozniacki and Wimbledon champion Angelique Kerber having already fallen, few major obstacles stand between Williams and a seventh title in NY. But she has never played Williams.
"That's a really big step for me". But the 28-year-old Latvian, who left the pro tour for 16 months ending in January of 2015 with recurring illness and injury, is 15-3 in recent matches.
Karolina Pliskova, projected to move as high as No.7 (depending on Osaka's result), gained a 4-2 lead but struggled to raise her level when Serena went from awful to deadly towards the end of the first set and lost 8 straight games from that moment.
"I could have s*** the bed [lost] in the first round, and that would have been really bad", said Stephens, who is projected to drop five spots to eighth in the world rankings.
"Still long way to go, I think", the Latvian said.
Stephens looked a step behind from the start of the match and failed to convert seven break points in the first set against Sevastova, which put her in a risky position against a woman who entered the match having broken her opponent's serve a tournament-leading 23 times.
"I was having a baby this time past year, so I've got nothing to prove".
"She still hasn't played many tournaments yet, but her level is there". I'm just taking it one at a time.
Bidding to reach her first Grand Slam semifinal since 2017 Roland Garros, Pliskova had to wait for the sixth game of the second set to hit her first ace of the match.
In response, Nike, who designed the outfit for the tennis star, replied with a succinct rejoinder: "You can take the superhero out of her costume, but you can't take away her superpowers". I think she's still the strongest. Other times, like the match today, I will try to be more consistent.