Rossi gets Yamaha return underway
Speaking before the weekend, Rossi had said he was excited to understand his potential after the first two winless seasons of his career on board the Desmosedici, and to see how he compared to champion team-mate Jorge Lorenzo.
"We are here to try and get the best results, and after two difficult seasons now we move forward," he said.
"It will be very important if we are able to do one or two dry days in [the] test to understand our potential, but I am happy [to join Yamaha].
"I'm very curious, but most of all I hope to become a problem for Lorenzo."
Bad weather has however restricted his first day of running, Rossi completing just 10 laps in the opening three hours.
That was still 10 more than Moto2 champion and new Honda works rider Marc Marquez.
Two days after his extraordinary win from the back of the field in his Moto2 farewell, the Spaniard has thus far opted to completely sit out the opening of the first post-season test.
Fellow Moto2 graduate Bradley Smith also cut a frustrated figure on the sidelines.
“This morning has been the biggest letdown of my career but there is absolutely nothing I can do about it," he said.
"When my MotoGP leathers arrived yesterday and I was measured and weighed for the bike it really hit home what a big day this was.
"This morning has been the worst conditions possible and if the track wasn't really soaking the team would have let me go out to get a feel of the bike. Frustrated is just how I feel on my big day but my chance will come – hopefully soon."
Andrea Dovizioso, the man he replaced at the Tech 3 Yamaha squad completed his first laps for Ducati, having agreed to join the works team in August.
Andrea Iannone, newly installed in Ducati's restructured satellite Pramac team, also opted to sit out the first three hours due to the rain and cold temperatures.