While the 19th seed Latvian may be disappointed with the outcome, Sevastovoa said she left her first Grand Slam semi-final with a sense of accomplishment.
Asked what positives she was talking away from Flushing Meadows, Sevastova said it was the realisation that she could "play with the best players in the world".
"I beat two top-10 players. I was playing (an) okay match today, maybe not my best tennis," Sevastova told reporters after the match. "Yeah, just knowing that I can be there, that I'm first time semi-finals.
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"I need couple of days to let it sink in. Maybe I'm more confident now."
Sevastova's performance at this year's tournament, during which she dispatched 2017 champion Sloane Stephens in the quarter-finals, marked the culmination of a long road back.
With persistent injuries plaguing her early years as a professional, she decided to retire in 2013.
But Sevastova came storming back to the sport in 2015 and clinched a U.S. Open quarter-final berth in 2016, her first appearance in the last eight at a Grand Slam. She repeated the feat last year.
Reflecting on Thursday's match, Sevastova said Williams was simply too tough to beat.
"When she's in front, it's tough to play," Sevastova said. "She stayed focused in the second set. I tried on her games, but she served well. I returned okay I think for my standards,"
"But when somebody serves 123 miles-per-hour ace, not much you can do."
(Editing by Greg Stutchbury)
