The world No.48 beat world No.1 and newly crowned Wimbledon champion Barty 6-4 6-3 in the biggest shock of the Games to date as the Australian fell apart in the blistering Tokyo heat.

Barty made a staggering 55 unforced errors to the Spaniard's 13, the Queenslander desperately out of sorts 15 days after clinching her second grand slam title.

"It was a tough day. A disappointing day. I can't lie about that," she told the Seven Network.

"I can't hide behind that fact that I wanted to do really well here. Today wasn't my day.

"Just loose. I knew I wanted to try to take the match on (and) it would be a fine line of not pushing too hard and not getting in the patterns I didn't want to get stuck in.

"(I was) too erratic."

The scorching, humid conditions had men's No.1 Novak Djokovic calling for matches to begin later in the day after his first-round victory on Saturday.

Although the schedule remained the same on Sunday, a concession was made with longer breaks at changes of ends as part of the extreme weather policy.

"It's brutal, like an Australian summer (but) I enjoy the heat and love playing out here in these conditions. Wasn't meant to be," Barty said.

It leaves Barty's medal hopes resting on the doubles, having advanced to the second round with Storm Sanders after a win on Saturday.

Her exit opens the door for a fairytale Naomi Osaka gold after she made quick work of her first match in nearly two months.

The Japanese superstar who lit the Olympic cauldron, eased past 52nd-ranked Zheng Saisai of China 6-1 6-4.

It was Osaka's first match since she withdrew from the French Open and sat out Wimbledon to take a mental health break while revealing she has dealt with depression.

Ajla Tomljanovic enjoyed a win on her Olympic debut, the Australian leading 7-5 3-2 before Yaroslava Shvedova retired.

Max Purcell made the most of his incredible rise from Tokyo spectator to last-minute call-up, beating world No.15 Felix Auger-Aliassime 6-4 7-6 (7-2).

Purcell had joined the team last week after Alex de Minaur tested positive to COVID-19 but was only due to play doubles before Andy Murray withdrew with injury hours before his match.

James Duckworth will join him and Australian teammate John Millman in the second round of the men's singles, the world No.77 continuing his prosperous year with a comeback 5-7 6-3 7-6 (7-4) win against Slovakia's Lukas Klein.

A late replacement in the singles for the injured Marton Fucsovics, Australian Luke Saville was beaten 6-2 6-4 by Hubert Hurkacz.

Australian Associated Press