Wimbledon champion Garbine Muguruza was bundled out of the Bank of the West Classic by Madison Keys on Saturday.
Number three seed Keys won 6-3 6-2, ending her Spanish opponent's nine-match winning streak in the process.
Muguruza had impressed on the way to the semi-finals of the California tournament as she sought to follow up her impressive Grand Slam success last month.
However, the top seed was punished for an off-colour performance by America's Keys, who wrapped up victory in 58 minutes.
Keys will play Coco Vandeweghe in Sunday's final after the American's straight sets victory over compatriot Catherine Bellis.
In the first career meeting between the two, number six seed Vandeweghe beat her 18-year-old opponent 6-3 6-1 to maintain an impressive run which has seen her win every set in this year's Stanford tournament.
Vandeweghe is the odds-on favourite at 4/5 with Sky Bet to win the final and Keys can be backed at evens.
In the Citi Open at Washington, Ekaterina Makarova will face Julia Goerges in Sunday's final at Rock Creek Park Tennis Center.
Russia's Makarova beat France's Oceane Dodin 3-6 6-3 6-4, while Goerges came from behind to win the all-German semi-final against Andrea Petkovic 5-7 6-4 7-5.
Makarova has been priced up Sky Bet as the 8/11 favourite to defeat Goerges, who can be backed at 11/10.
