Sevastova said she'd watch Tuesday night's match with interest to see who emerges as her last-four foe.
"I had done not many goals", she said of her plans upon returning.
For a spot within the semi-finals of the event, Stephens will compete with the tennis participant from Latvia Anastasiya Sevastopol, which is within the fourth spherical beat Ukrainian Elina Svitolina.
Serena Williams and Sloane Stephens can give the United States two U.S. Open semifinalists, a year after an all-American women's final four. "I think she lost also her nerves a little bit, it's normal".
"But in the end it's unbelievable, yes? So after I stop at some point I will look at it and I will be proud of myself, for sure".
"The last games, it was good that I played in the shade".
Williams reached her 36th Grand Slam semifinal and her 12th at Flushing Meadows, where she is a six-time champion.
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"It was an incredible journey this three, four years", Sevastova told reporters after the match, a repeat of last month's Rogers Cup semi which Stephens won in straight sets.
Sevastova is one of the wiliest players on tour, mixing up slices, spins and angles and generally making life very uncomfortable for a player like Stephens who feeds off consistency of pace.
Stephens had used some obscene words out of desperaiton for not being able to execute her shots with pinpoint precision like in the other three rounds to win the match. There were only a few more games that Sevastova had to do in sealing the round and Stephens' unforced errors were part of the problem which kept appearing like an unwanted guest at a celebration. But her serve was the bigger problem today as the third seed was broken five times in the 84-minute match.
Stephens saved two match points but not a third and later revealed she has been suffering from a sinus infection for two days. Even when I did have my break opportunities, I just wasn't playing the points well at all. It just was a really tough day. "But on the same time, not make too many mistakes", said the 24-year-old Thiem. "The better player won".
Six-time US Open victor Williams is seeded 17th as she battles for her first Grand Slam title since the birth of daughter Olympia during last year's tournament. Stephens played the second set with the sun pounding down on her, and the announcers said that only three games all tournament had been won in the sun.
"And obviously defending a title is very hard, very difficult".