Brazil held off an Egypt fightback to open their Olympic campaign with a 3-2 win at the Millennium Stadium.
Brazil came into the Games as favourites for gold and charged into a 3-0 lead inside half an hour through Rafael, Leandro Damiao and Neymar.
Mohamed Aboutrika gave Egypt hope of an unlikely comeback and despite Mohamed Salah's 76th-minute strike, they failed to pull off a shock result.
The other Group C game saw Belarus beat New Zealand 1-0 in Coventry.
Brazil are hoping to win their first football gold and turned on the style during the opening half-hour in Cardiff.
Manchester United right-back Rafael latched onto a through-ball from new Chelsea signing Oscar to beat one man before firing in a 16th-minute opener.
Oscar then went round the keeper to pull the ball back for Damiao to double Brazil's lead in the 26th minute and Egypt looked dead and buried four minutes later when Neymar found Hulk on the left before heading home the return ball.
Captain Aboutrika pulled one back in the 52nd minute after Meteab Emad had hit a post and substitute Salah produced a well-taken strike from just inside the box.
But Brazil saw out the victory to top the group with Belarus, who they play in Sunday's double-header at Old Trafford.
Belarus dominated the first half against New Zealand, with Sergei Kornilenko firing wide in the opening minute, before the Kiwis replied with a header from West Brom striker Chris Wood testing Belarus keeper Aleksandr Gutor.
New Zealand keeper Michael O'Keefe then denied Renan Bardini Bressan and Dmitry Baga, but he could not prevent Baga heading home on the stroke of half-time.
Although New Zealand came out stronger in the second half, they were unable to break through the Belarus defence.