But rookie Johann Zarco flew round the first six laps and into the lead before crashing out at Turn Two.
And while others struggled in the slippery conditions, Rossi was masterful and managed to nurse his Yamaha bike home in third behind pole-sitter Maverick Vinales and Andrea Dovizioso.
“For sure I’m pretty happy,” Rossi said.
“If I had to bet on myself to make the podium today I wouldn’t have bet. Or 10 euros or something.
“But we never gave up. We tried the maximum.
“We tried three or four different settings this morning but anyway it was bad, so I just said ‘go’.
“I don’t know what happened but I enjoyed it.
“I was good in braking and I hope this race will help us understand enough to improve.”
Defending world champion Marc Marquez finished third in the same race last year but could only manage fourth on his Honda machine, which appeared to be struggling for power at times.
Meanwhile, Britain’s Scott Redding brought his OCTO Pramac bike home in seventh unlike fellow countryman Cal Crutchlow who crashed twice before retiring after just four laps.