Hosts Japan beat crisis-hit South Africa 1-0 in their opening game of the Olympics.
The match was in doubt when 18 South African players had been identified as close contacts after two of their team-mates and a video analyst tested positive for coronavirus on Sunday.
They were the first confirmed cases among Olympic teams at the athletes village but 17 players trained on Wednesday ahead of the game.
Real Madrid's Take Kubo compounded South Africa's woes with a second-half winner in the Group A game.
Richarlison's first-half hat-trick inspired defending champions Brazil to a 4-2 win over Germany in a re-run of the 2016 final.
The Everton striker completed his treble after just 30 minutes but Brazil were given a scare when Nadiem Amiri and Ragnar Ache scored after Maxi Arnold was sent off for Germany.
But Paulinho wrapped up the Group D victory in stoppage time for Andre Jardine's side.
Mexico stunned France 4-1 to take early control of Group A, with two goals in eight second-half minutes from Alexis Vega and Sebastian Cordova putting them in control.
Andre-Pierre Gignac's second-half penalty gave France brief hope but Eduardo Aguirre and Uriel Antua netted in the final 10 minutes to seal a comfortable win for Mexico.
Burnley's Chris Wood scored with 20 minutes remaining as New Zealand beat South Korea 1-0 in Group B – the men's first-ever Olympics victory – with Honduras beaten 1-0 by Romania in the other game.
Egypt and Spain, who lost Real Madrid's Dani Ceballos to an ankle injury, drew 0-0 in Group C while 10-man Argentina suffered a 2-0 defeat to Australia.
AC Milan's Franck Kessie grabbed the winner for Ivory Coast after they came from behind to beat Saudi Arabia 2-1.
Salem Al-Dawsari opened the scoring after 10 minutes but Abdulelah Alamri's own goal levelled before Kessie struck in the second half to win the Group D opener.
