The two big men battered each other to near exhaustion in the humidity of Arthur Ashe Stadium but it was Del Potro who proved the stronger in a 6-7 (5/7) 6-3 7-6 (7/4) 6-2 victory that lasted three and a half hours.
Osaka will face either 14th-seeded Madison Keys or No. 30 Carla Suarez Navarro in the first major semifinal appearance for a Japanese woman since Date reached the final four at Wimbledon in 1996. It's tough to play, so hot in here.
Sevastova, who avenged last year's quarter-final loss to Stephens in NY to reach her first grand slam semi-final, converted five of her eight break points and saved seven of the nine against her on the Arthur Ashe Stadium court. The shot that earned her the decisive break with Stephens serving at 3-4 in the second set was a sidespin backhand slice that landed in the midcourt - the last place anyone, including Stephens, expected to see a ball.
"I had done not many goals", she said of her plans upon returning.
'Obviously today doesn't factor into that, but, yeah, I hope that I can go to Asia and win some more matches and hopefully make the year end and hopefully keep going as far as I can'.
Rafael Nadal could feel grateful that it is 48 years since the US Open introduced, uniquely among the Grand Slams, a deciding set tiebreak.
AP Serena Williams celebrates after defeating Karolina Pliskova in the quarterfinals of the US Open
The win was ultimately a grudge match for Sevastova, who lost to Stephens in last year's US Open quarterfinals. Thiem is a top-10 player himself (he's now ranked No. 9), a star in his own right who some consider to be the heir apparent to Nadal.
Playing in her third straight quarterfinals at the US Open - she had lost at that stage the previous two editions-the 28-year-old Sevastova looked primed for the big stage.
That both players didn't completely collapse from the extreme heat is a triumph in itself - play at the junior level was suspended due to the conditions, but the adult circuit has a higher heat threshold to suspend play.
Jamie Murray angrily hurled threw his racket to the ground after he and Brazilian partner Bruno Soares last night missed out on the chance to make the semi-finals of the doubles event. Sevastova, meanwhile, was a ideal two-for-two.
What has to be done is keeping her game plan and strategies in tact, practicing a bit of change ups and staying positive. I just enjoying playing tennis again. "I think I try not to put too much emphasis on defending the title, just try to come in and win as many matches as possible".
Though Stephens landed 40 of 49 first serves (82 percent), she won just one of her second-serve points in the one-hour, 23-minute elite-eight matchup.
He added "I'm so happy to reach another semi-final in my favourite tournament". So I'm not going to dwell on it.
Over 500 flights cancelled as Typhoon Jebi bears down on Japan
A auto lies upside-down along a road after being blown by typhoon Jebi's strong winds in Osaka on September 4, 2018. The storm also did serious damage to Kansai International Airport, one of Asia's most important airports.