There were some question marks surrounding Marta ahead of the tournament.
The 35-year-old, who has been named the world's best player six times, needed just nine minutes to show his goal danger.
After a nod in the crossbar by Debinha, the ball went out to Marta who cut in 1-0.
With a quarter of an hour left in regular time, Marta pulled in from outside and saw the ball sneak in at the post for 3-0.
When Alves put 4-0 on a hard-hitting penalty, Sundhage and the other leaders danced ring dance wearing mouth guards.
In the final minutes, Brazil broke high and started a quick counterattack.
The ball was played in the middle to Beatriz who stuck his foot out and angled in 5-0.
China was pale but had a nod in the crossbar and a post shot in the second half.