The Japanese claimed a shock victory at the Indian Wells Masters last week and was then handed a stand-out tie against the American 23-time Grand Slam champion in the Florida city.

Williams, who is on the comeback trail after taking 13 months off to give birth to her first child, was no match for Osaka who came through 6-3, 6-2 on the hard courts of Crandon Park.

Both players are unseeded but Osaka's recent exploits have seen her move up to number 22 in the world.

"I was extremely nervous coming on to the court," said Osaka.

"I don't know if anybody knows this but Serena is my favourite player."

Elsewhere in the draw, Puerto Rico's Olympic champion Monica Puig made it through as she saw off Australian Sam Stosur 6-3, 6-4.

Belgium's Kirsten Flipkens defeated Romania's Mihaela Buzărnescu 6-2, 6-3 to set-up a meeting with Britain's defending champion Johanna Konta in round two, when the seeded players come in.

In the men's draw, Croatia's former Davis Cup champion Ivo Karlović was knocked out in round one 7-6, 7-6 by Canada's Vasek Pospisil.

France's Jeremy Chardy, also a Davis Cup winner last year, defeated Brazil's Rogério Dutra Silva 6-3, 7-6.

Australia's qualifier John Millman came from behind to oust Germany's Peter Gojowczyk 2-6, 6-4, 6-4.

More first round games will be played tomorrow, with the women's second round also beginning.

Switzerland's Roger Federer is the defending men's champion at the tournament.

The event is a Masters competition on the Association of Tennis Professionals Circuit and a Premier Women's Tennis Association meeting.