Williams is a heavy [1.23] favourite to make the final, and I make this a little short against a rapidly improving opponent - Sevastova has won 15 of her last 17 matches and this year on hard court, has held 66.1% and broken 42.5% (combined 108.6%).
It was just the latest in what's been a dominant run for Osaka.
Nishikori allowed his advantage to slip, missing a break point that would have given him a 5-1 lead and then being pegged back to 4-4.
Her compatriot Nishikori had rather a more hard time of it in his matchup against Maric Cilic, taking five sets to oust the Croat, 2-6 6-4 7-6 4-6 6-4.
"So I knew that I was going to have to play really well but also know that she's going to come up with some great shots and defend really well". Meaning until she wins another major title.
The 20-year-old trounced Lesia Tsurenko 6-1 6-1 to become the first Japanese woman since Kimiko Date at Wimbledon in 22 years to advance to the last four of a grand slam.
The No. 14 seed was last year's US Open runner-up, losing to Stephens in the final, the only Grand Slam final appearance of her career. "Especially after coming from injury, I think I'm enjoying this challenge".
She has made it to the final four at three of the past five Grand Slam tournaments but is still seeking her first major championship.
Tropical Storm Gordon hits Gulf Coast
Lance Murphy, who referred to Gordon as a "party storm", said, "We just live through it and hope we can open again tomorrow". The disturbance is expected to reach hurricane status before it hits the Gulf coast Tuesday evening.
Spanish 30th seed Suarez Navarro, playing in her second US Open quarter-final, has made this stage by seeing off French sixth seed Caroline Garcia as well as former champion Maria Sharapova in the fourth round.
"Unfortunately during this tournament I had many issues with my health, and today was not my day obviously".
But after laboring through the heat in her previous match, Tsurenko said she was sick Wednesday, waking up with a sore throat and not breathing well.
In 2017, Williams played the Australian Open and won the title without dropping a set - and perhaps that might have been impressive enough, if not for the fact that at the time she was also two months pregnant with her now one-year-old daughter, Olympia.
Osaka had consecutive 50-minute matches earlier in the tournament, including a 6-0, 6-0 thrashing of Aliaksandra Sasnovich in the third round.
Keys had the chance to close out the set on Suarez Navarro's racquet at 5-4 and while she was denied at the first time of asking, she did not waste her second opportunity.
Keys punched her ticket with a victory over Carla Suarez Navarro on Wednesday.
It was another hot afternoon Wednesday, with temperatures in the high-80s (30s Celsius) but feeling some 10 degrees hotter with the humidity. Osaka put it best after her quarterfinal win: "Sometimes I go insane and then I start hitting everything". "I didn't want to show this kind of game in front of this big crowd, but unfortunately I'm just not able to play now". "I think we're pretty similar in that sense".