Brazil held off Australia to open their Group B campaign with a narrow 75-71 win.
Patty Mills did his best to keep the Boomers in touch, finishing with 18 points, but Brazil's big men proved more than Australia could handle.
Brazil's Tiago Splitter said afterwards: 'We didn't play at 100% but we are happy because we fought to win a good game.'
Nigeria almost squandered a 19-point lead but hung on to beat Tunisia 60-56.
Nigeria used their superior size to dominate the first half and led 29-10 midway through the second quarter. Tunisia found their shooting rhythm but were edged out in an entertaining game.
In the end, Al-Farouq Aminu led Nigeria with 15 points and nine rebounds.
The USA lived up to their dominant reputation with a 98-71 win over France in their opening game.
Although the French, who have plenty of high-calibre talent in their squad, were able to match them for the first quarter, the defending champions quickly began to wear them down thereafter.
A score gap of one at the end of the first had become a deficit of 16 by half-time, before nearing 30 in the final minutes.
European champions Spain got their medal challenge under way with a 97-81 victory over China made comfortable by a late surge.
Pau Gasol scored 21 points while Serge Ibaka added 17, Juan Carlos Navarro 14 and Jose Calderon 12.
Great Britain's long-awaited return to action in Olympic men's Basketball ended in a 95-75 defeat as they were on the wrong end of an explosive display from Russian star Andrei Kirilenko.
Britain battled hard but committed a series of costly turnovers and were made to pay by Kirilenko, who finished with 35 points.
Britain's own star Luol Deng shone brightly as he poured in 26 points and Pops Mensah-Bonsu added 22, but Russia always had an answer.
Argentina broke through the 100-point barrier as they overcame Lithuania 102-79 in the late game.