Britain's Johanna Konta beat seven-time Grand Slam champion Venus Williams to win her first WTA title in Stanford.
Konta, 25, won 7-5 5-7 6-2 in California to join Heather Watson as a British winner on the WTA Tour in 2016.
"It's quite an incredibly humbling experience," said the British number one. "It's a validation of all the hard work you've already put in."
Sue Barker was the only previous British winner in Stanford, beating Virginia Wade in the 1977 final.
Konta said: "I wanted to leave it all out there, but also absorb everything that I could possibly reinvest in my career moving forward.
"I've played her twice before and knew I'd be playing a magnitude of experience. Venus Williams doesn't need an introduction."
British number one Konta - ranked 147th last June - will rise from 18th to a career high 14th when the new world rankings are confirmed on Monday.
She reached the semi-finals of the Australian Open in January, and again at Eastbourne in June.
Watson became the first Briton for 24 years to win a WTA title in Osaka four years ago, and has since added victories in Hobart (2015) and Monterrey (2016).
Konta's success comes at a higher level, however, with Stanford among the WTA's 'Premier' tournaments and boasting former winners that include Martina Navratilova, Chris Evert, Monica Seles, Martina Hingis and Serena Williams.
Konta caps stunning rise with trophy
After overpowering world number 12 Dominika Cibulkova in Saturday's semi-finals, Konta looked on course for a similarly confident win at 7-5 4-1 up in the final.
Williams, 36, was wobbling when she double-faulted to fall two breaks down in the second set but the five-time Wimbledon champion came storming back.
With nerves gripping Konta, the American took six out of seven games to force a decider.
However, it was the Briton who proved the stronger on a fiercely hot afternoon and she again forged a commanding lead in the final set.
The tension was apparent as Konta tried to serve out the match and she had to fend off three break points in a dramatic game before converting her third match point.
"It was about keeping things in perspective, and understanding there'd be ebbs and flows in the match," said Konta. "Every single point was a battle, and I tried to win as many battles as possible.
"The simpler you keep things, the more clarity you have, and the less dumb you play!"
Williams, who has 49 WTA singles career titles and reached this year's Wimbledon semi-final, said: "She played at such a high level today. She saved her best tennis for the final, which is what you want to do.
"She plays really well against me, so maybe she comes out and doesn't feel any pressure and just swings for it. I tried to stay in there and fight. What can I say but give her credit."