Sevastova defeated Gauff in three sets 6-3, 5-7, 6-4 in the opening round to hand the 16-year-old star her earliest exit from a Grand Slam tournament in four appearances. Last year, Gauff reached the third round at Flushing Meadows before losing to 2018 champion Naomi Osaka.
"I could've played better today, but I'm just going to get back to work and get ready for the French Open," said Gauff, who had 13 double-faults.
Sevastova had been only 1-8 in singles play this year but earned another breakthrough win in New York. She was a semifinalist at the U.S. Open two years ago.
Earlier Monday, top-seeded Karolina Pliskova beat Anhelina Kalinina in straight sets to advance to the next round. Pliskova had a 26-7 edge in winners.
Three-time Grand Slam champion Angelique Kerber defeated Ajla Tomljanovic 6-4, 6-4. Kerber, who won the 2016 U.S. Open but lost in the first round last year, is the No. 17 seed in this year's event.
On the men's side, fourth-seeded Stefanos Tsitsipas cruised into the second round of the spectator-free U.S. Open with a straight-sets victory over Spain's Albert Ramos-Vinolas. The Greek player needed only one hour and 38 minutes to seal a 6-2, 6-2, 6-1 win.
Tsitsipas, 22, will play American wildcard Maxime Cressy in the next round.
Elsewhere in the men's draw, No. 5 seed Alexander Zverev beat Kevin Anderson in four sets. No. 12 seed Denis Shapovalov required treatment on a back problem but managed to edge Sebastian Korda in four sets.
The U.S. Open is the first Grand Slam event to take place since the beginning of the coronavirus pandemic. The tournament is being played without fans -- and some of the sport's biggest stars -- in a modified bubble.
Despite the U.S. Tennis Association's efforts, France's Benoit Paire became the first player in the field to test positive for COVID-19. According to USA Today Sports and French newspaper L'Equipe, Paire will miss the remainder of the event.
The U.S. Tennis Association announced the positive test in a statement Sunday but didn't identify the player. The statement said the infected player is asymptomatic.
Paire, 31, was scheduled to begin the tournament with a match against Kamil Majchrzak on Tuesday. According to the outlets, Marcel Granollers will replace Paire.