Four-time Grand Slam champion Osaka, who has fallen to 78th in the world so is unseeded in California, grew into her match with Sloane Stephens and ended with eight breaks of serve.

The 3-6, 6-1, 6-2 win is Osaka's first over American Stephens, the 2017 US Open champion.

Next up for Japan's Osaka - who won this Women's Tennis Association 1000 event in 2018 - is neutral player Veronika Kudermetova.

Kazakhstan's Yulia Putintseva cruised into the second round, beating American Ashlyn Krieger 6-3, 6-2, while Tereza Martincová was even more dominant.

Martincová beat Britain's Heather Watson 6-2, 6-1, and will play defending champion Paula Badosa of Spain in the next round.

Ukrainian teenager Marta Kostyuk is also through, following a three-hour, two-tiebreak match against Maryna Zanevska.

Kostyuk triumphed 6-7, 7-6, 7-5 and will play anther Belgian, Elise Mertens, in round two.

The all-American clash between Amanda Anisimova and Emma Navarro went the way of Anisimova, who did not face a single break point in a dominant 6-2, 6-2 win.

Anisimova is set to play Canadian Leylah Fernandez next.

The men's first round also began today.

Italy's Fabio Fognini fought back to beat Spain's Pablo Andújar 3-6, 6-3, 6-3 in one of the more eye-grabbing matches.

An absent Serbian loomed large over the build-up, but one who is playing, Laslo Đere, overcame South Korean number one Kwon Soon-woo while also coming from behind.

Đere won 2-6, 6-3, 6-4, despite being broken as many times as his opponent.

There were mixed fortunes for Australians Nick Kyrgios and Thanasi Kokkinakis.

Kokkinakis went down 6-3, 6-4 versus American Sebastian Korda. 

Kyrgios, on the other hand, produced 12 aces in a 6-4, 6-0 victory versus Sebastian Baez from Argentina.