Brazil 's campaign started with a rematch of the 2016 final it won on home soil against Germany on PKs, and prevailed 4-2 after taking a 3-0 lead.
Mexico, the gold medalist in 2012 London, won 4-1 against a rag-tag French team, whose goal came from a penalty kick converted by André-Pierre Gignac, one of its three overage players along with 2018 World Cup champion Florian Thauvin and Montpellier's Teji Savanier.
The 35-year-old Gignac, who's played for Mexican club Tigres since 2015 and was picked for the Liga MX selection that faces the MLS All-Stars Aug. 25. Thauvin joined him at Tigres from Marseille last May.
In Brazil's win, 24-year-old Everton forward Richarlison scored a hat trick before Germany got on the board. Richarlison's having a busy summer. Eleven days ago he started for Brazil in its 1-0 loss to Argentina in the Copa America final.
The Argentines, gold medal winners in 2008 and 2004, lost their opener, 2-0, to Australia.
Honduras, which knocked the USA out of qualifying, fell, 1-0, after gifting Romania an own goal.
The second round of group play takes place on Sunday, when Mexico faces host Japan, Honduras will aim to bounce back against New Zealand, and defending champion Brazil takes on Ivory Coast.
Thursday, July 22
Group A Mexico 4 France 1 (Goals: Vega 47, Cordova 55, Antuna 80, Aguirre 90+1; Gignac, pen. 69)
Japan 1 South Africa 0 (Goal: Kubo 71)
Group B New Zealand 1 South Korea 0 (Goal: Wood 70)
Romania 1 Honduras 0 (Goal: own goal, 45+1)
Group C Egypt 0 Spain 0
Australia 2 Argentina 0 (Goals: Wales 14, Tilio 80)
Group D Ivory Coast 2 Saudi Arabia 1 (own goal 39, Kessie 66; Al Dawsari 44)
Brazil 4 Germany 2 (Goals: Richarlison 7, 22, 30, Paulinho 90+5; Amiri 57, Ache 84)