Serena Williams defeats Karolina Pliskova in straight sets (6-4, 6-3) and ends the match with an ace, advancing to the semifinals. (0:53)

NEW YORK -- If you haven't been paying attention for some time, the women's semifinals at the US Open might seem like it's Serena Williams' to lose. After all, among the other semifinalists, only Madison Keys has been in even one Grand Slam final.

But this US Open represents new and still uncharted territory for Williams, and nobody -- least of all Serena -- will say she's back until ... she's back. Meaning until she wins another major title.

Here's how the semis stack up:

No. 17 Serena Williams versus No. 19 Anastasija Sevastova (first meeting)

Sevastova looks to be in deep trouble from the get-go, because she has never felt the weight of Williams' shots or the unrelenting pressure her aggressive play applies. Sevastova is a fine player, but she could find herself down a set and 3-0 before she even knows what happened.

Serena Williams is one win away from making her second consecutive Grand Slam final. Robert Deutsch/USA TODAY Sports

Williams wants this title so badly she can probably taste the silver polish on the trophy. To win her record-tying 24th Grand Slam singles title as a new mother will add a transcendent chapter to her already glowing biography. She knows she missed a great opportunity at Wimbledon. As Williams said after her quarterfinal win here: "I still know that no matter whether I'm in the semifinals or the finals, I have a really long way to go to win that. Again, that proved to be true at Wimbledon. I'm just taking it one at a time. Literally."

That Williams is being so cautious is more bad news for Sevastova. Williams is leaving nothing to chance. The statistics show she's backing up her prudence with power. Williams leads the tournament with 60 aces, almost twice as many as her nearest rival, Keys. Williams has won 78 percent of her first-serve points, compared to Sevastova's 68 percent.

Sevastova has never been ranked higher than No. 15. This is her deepest run at a Grand Slam. But the 28-year-old Latvian, who left the pro tour for 16 months ending in January 2015 with recurring illness and injury, is 15-3 in recent matches.

A versatile right-hander who makes frequent use of slice, Sevastova will most likely try to take pace away from Williams. Sevastova will employ her excellent drop shot to change the pace as well as in an effort to move Williams around the court. The strategy paid off against Sloane Stephens, but the defending champion was off her game amid an afternoon of more oppressive heat and humidity.

It promises to be much cooler Thursday evening, and it's unlikely Williams will be out of sorts with an epochal achievement so close at hand. A game plan built on counter-punching and finesse is always vulnerable to a player like Williams. She will step in and powder the ball, pushing Sevastova back and making it hard for her to bring her finesse into play.

Prediction: Williams is focused and sharp. It's hard to imagine Sevastova getting more than two games per set.

No. 14 Madison Keys vs. No. 20 Naomi Osaka (Keys leads series, 2-0)

play 0:17 Osaka beats Tsurenko in straight sets to reach US Open semis Naomi Osaka advances to the US Open semifinals with a straight-sets victory over Lesia Tsurenko.

Both of these women have undergone a painful learning experience they hope never to repeat on Ashe. The one Naomi endured was inflicted by Keys here in 2016, when the American came roaring back from a 5-1 deficit in the third set of their third-round clash to beat then-18-year-old Osaka. Last year, it was Keys who froze up in the final against Stephens in the tournament final. Keys was barely able to hit a ball in the court.

All that seems to be behind them now. They've both played commanding tennis from the baseline this year, relying on similar tools. Osaka put it best after her quarterfinal win: "Sometimes I go crazy and then I start hitting everything. Other times, like the match today, I will try to be more consistent. But I think [basically] she's more of a power player than I am. I would say it's a similar style."

Prediction: Statistically and stylistically, there's little to choose between the two. But this is Keys' fourth Grand Slam semifinal. She'll win, but it will be close.