The 19-year-old had led Novak Djokovic two sets to love in the French Open round of 16 before eventually retiring hurt in the fifth set and was fancied to topple the Australian on the Ariake Tennis Park hard court on Saturday.

But the form line didn't account for the Brisbane workhorse's tremendous resolve, particularly while wearing Australian colours in his second Games appearance.

He broke Musetti for a 5-3 lead and then served out the first set, before twice losing his serve after gaining breaks in the second.

Millman held his nerve to win a tight service game at 3-4 before Musetti's frustrations eventually got the better of him in a 96-minute contest played in hot, humid conditions.

Millman recorded the Games' first 6-0 6-0 win to open his account in Rio five years ago, before blowing leads in both sets against eventual bronze medallist Kei Nishikori in the second round.

Established as a top-50 player since then, Millman is leading the Australian men's charge after the withdrawal of Nick Kyrgios and Alex de Minaur.

His second-round opponent is the in-form Alejandro Davidovich Fokina.

If Millman can negotiate a way past the Spaniard he will likely come up against world No.1 Djokovic in the third round.

Meanwhile Samantha Stosur, in an Australian tennis record fifth Games appearance, was comprehensively beaten 6-4 6-2 by Elena Rybakina.

Her focus will now switch to the doubles alongside Ellen Perez, while Australia's other women's pair of Ash Barty and Storm Sanders will begin their doubles campaign later on Saturday.

World No.1 Barty, fresh off her Wimbledon triumph, begins her singles campaign against Spain's Sara Sorribes Tormo on Sunday.

Australian Associated Press