Temperatures soared to 39 degrees Celsius (102 degrees Fahrenheit) as Armstrong, among a field of 33 riders, raced through undulating hill country in rural South Australia state.
"That was tough, and this is the day they said is going to be the easy day,'' Armstrong said.
Defending tour champion Andre Greipel of Germany won the stage, a 140-kilometer (86-mile) ride from Norwood to Mawson Lakes north of Adelaide, in a bunch sprint.
"I feel awesome,'' Greipel said. "It was hot today and everyone had to suffer, but today was really great.''
Thousands of spectators lined the streets of small towns or stood outside remote farmhouses to cheer as Armstrong passed.
The six-stage ProTour race ends on Sunday.
OneIndia News (With inputs from Agencies)