Johanna Konta won 12 games in a row to brush aside 23-time Grand Slam winner Serena Williams 6-1 6-0 in San Jose.
Konta, ranked 48th in the world, took less than an hour to book her place in the second round of the Silicon Valley Classic.
Her 53-minute rout of the younger Williams sister started with two games going to serve before the Konta won five on the bounce.
The second set was even more clinical than the first, as Konta won two service games to love and dished out the bagel on the first of two break - and match - points.
The WTA said it was the most one-sided loss of Serena Williams' career, in which she served seven double faults and made 25 unforced errors compared to Konta's nine.
The British number one will go on to play Sofia Kenin in the next round.
Speaking on court after the win, Konta said: "It's an absolute privilege for me to be on the same court as her.
.@JohannaKonta stuns Serena Williams, 6-1, 6-0!August 1, 2018
"She obviously wasn't playing anywhere near her best level but I was just trying to play the match on my terms and do what I can out here and put aside the incredible champion she is and play the player on the day.
"I felt that I did better than her on the day, but it's still a humbling experience for me to be out here with her.
"I had to stay very much on every single point because against her it can turn around at any point."