San Diego, California - Despite being in red-hot form, US tennis star Coco Gauff crashed out of the Silicon Valley Classic at the quarterfinals stage on Friday after word No. 4 Paula Badosa stormed to a 7-6 (7-4), 6-2 victory.

Usually one of her stronger weapons, Gauff's serve was a liability on Friday, with six double faults in the first set alone to drop the opening frame despite breaking Badosa twice.

In the second set, while Badosa was converting 92% (11-of-12) of her accurate first serves into points, Gauff's figure was down at 53% (eight-of-15) as the Spaniard's return game was at its best.

The sixth-seeded 18-year-old was coming off the back of a stunning 6-4, 6-4 win over Naomi Osaka in the previous round and was the favorite for this matchup.

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Badosa will meet seventh seed Daria Kasatkina in the semi-final after the Russian beat Belarus' Aryna Sabalenka 4-6, 7-5, 6-0.

Shelby Rogers, who is unseeded, also pulled off a big upset by beating her fellow American and world No. 22 Amanda Anisimova in straight sets, 6-4, 6-4. She will face Russian player Veronika Kudermetova in the semifinals.