Johanna Konta inflicted the heaviest defeat of Serena Williams’s career with a 6-1, 6-0 win over the 23-time grand slam champion in San Jose.
The British No 1 took only 53 minutes to book her place in the second round of the Silicon Valley Classic.
Williams, who was playing her first match since losing to Angelique Kerber in the Wimbledon final last month and only her fifth tournament since giving birth last year, started the encounter by holding serve but did not secure another game.
Konta claimed 12 in a row and broke Williams to wrap up victory on the first of two match points.
Williams had never lost a match without winning at least two games since turning professional in 1995.
The 36-year-old made 25 unforced errors compared to Konta’s nine and served seven double faults.
Konta was sharp, firing six aces and smacking 17 winners to Williams' nine on Tuesday
“I tried to put aside the incredible champion Serena is and just play the player of the day,” said Konta, who will hope the victory represents a turning point after a disappointing season in which she has dropped to 48 in the world rankings.
“She obviously wasn’t playing at her best level, nowhere near it, and I really just tried to play the match on my terms.
I felt I did better than her on the day but it’s still a humbling experience to be out here with her.”
Williams said: “I think she played well in the second set and I wasn’t sharp at all in the first set. She got confident and clearly ran away with it.
“I know I can play a zillion times better so that kind of helps out. I have so many things on my mind I don’t have time to be shocked about a loss that clearly wasn’t at my best.”
Konta will play the 19-year-old American Sofia Kenin in the next round.
Her fellow Briton Heather Watson will meet Venus Williams after a 6-4, 3-6, 6-4 win over another American teenager, Claire Liu.