Gauff won five consecutive games to go up 5-1 in the second before struggling to close the victory. Osaka, the 2018 and 2020 US Open champion, was playing in her first tournament since falling in the first round at the French Open back in May. She had been sidelined recently due to an Achilles injury.

Next up for Gauff is a quarterfinal match against second-seeded Paula Badosa of Spain.

In other action in San Jose, Shelby Rogers delivered 10 aces and won 76.5 percent of her first-serve points while rolling to a 6-1, 6-3 upset of top-seeded Maria Sakkari of Greece. Rogers had 16 winners and eight unforced errors and needed just 72 minutes to finish off Sakkari. Rogers will face fellow American Amanda Anisimova in the quarterfinals.

Fourth-seeded Aryna Sabalenka of Belarus posted a 5-7, 6-1, 7-5 win over Caroline Dolehide, and No. 9 Veronika Kudermetova of Russia beat American Claire Liu 6-2, 7-5.

Citi Open

Second-seeded Emma Raducanu of Great Britain saved nine of 13 break points while outlasting Colombia's Camila Osorio 7-6 (5), 7-6 (4) in a marathon second-round contest at Washington.

Raducanu and Osorio battled for two hours 50 minutes in the sweltering heat - the longest WTA straight-sets match of 2022. Raducanu will face Liudmila Samsonova in the quarterfinals after the Russian defeated Australia's Ajla Tomljanovic 4-6, 6-3, 6-2.

China's Xiyu Wang rallied for a 4-6, 7-5, 6-1 win over Croatia's Donna Vekic. The scheduled match between fourth-seeded Victoria Azarenka of Belarus and Tereza Martincova of the Czech Republic was postponed due to lightning in the area.