Konta was out of sorts in Arthur Ashe Stadium and she paid the price as world No 48 Sevastova won 6-4 7-5 to continue her remarkable run.
The result ends Britain’s interest in the women’s singles at Flushing Meadows while Sevastova advances to her first major quarter-final. She awaits the winner of the match between Madison Keys and Caroline Wozniacki.
Three years ago Sevastova gave up tennis, fed up with injuries and poor form, and her only time spent with a racket came when giving lessons down at her local club.
Underwhelmed, however, by a college course in management, she returned in January 2015, initially at a lower level, and this year in the grand slams.
The 26-year-old’s wily style of play was enough to see off French Open champion Garbine Muguruza in round two and it upset Konta, who played in fits and starts but just never found her rhythm.
“It was hard moments in 2013,” Sevastova said on court.
“I was injured on and off, it was tough to play, it was not any fun and kind of depressing. But now I’m back.”
Andy Murray expressed his satisfaction at playing poorly but still grinding out victory over Paolo Lorenzi on Saturday and perhaps Konta must now learn similar powers of resilience.
Fatigue may have played a part – she admitted she was still recovering from her on-court collapse in the second round – but it was the British No 1’s usually reliable ground shots that cost her dear.
Konta made 34 unforced errors, more than five times the six she made in the previous round, with 16 coming on her favoured backhand side, while her serve was broken seven times.
Credit must also go to Sevastova, who won 15 of the opening 20 points and could have pulled 4-0 clear had she closed out a second break when up 0-40. A superb Konta drop-shot, however, appeared to spark the Briton into life and she came back, holding, breaking and then drawing level for 3-3.
No sooner had the Briton found her groove, however, than she lost it again, playing a dreadful service game to be broken again as Sevastova clinched the set.
The slump continued in the second as two Konta double faults gave away a break and Sevastova raced into a 4-1 lead. The Latvian blasted one short forehand straight into her opponent’s body and then twice had the chance for a knock-out, opening up match points at 5-3 and 5-4.
Konta saved both and when a flustered Sevastova slapped a simple backhand into the net for 5-5, a comeback looked on the cards.
It was quickly snuffed out, however, as Sevastova dug deep to hold and when a Konta forehand spiralled out, her victory was assured after one hour and 42 minutes.