Rossi was third and fourth in the two Friday practice sessions at Jerez, as his Yamaha team-mate Jorge Lorenzo led the way.
Though happy with his position, Rossi was concerned about tyre management.
"I'm more worried about the rhythm," he said.
"I wasn't very confident with the hard rear tyre in practice. I tried it but unfortunately the hard rear doesn't have enough grip on the very edge.
"I'm quite worried if I have to race with the soft because with this temperature it will become quite difficult to do the 27 laps."
Lorenzo was content with the soft tyre, although he will try the hard in Saturday practice.
Yamaha had also set the pace at Jerez in pre-season testing, but Rossi believes Honda's Dani Pedrosa will be a serious threat in the race.
Pedrosa was just 0.003 seconds behind Lorenzo in the morning.
"Maybe we have some advantage compared to the Honda in the colder conditions, like in the test," said Rossi.
"But anyway Dani is very fast here.
"I think that for Sunday, Dani and Jorge are the riders with more of a chance to win."
The two works Yamahas of Lorenzo and Rossi, factory Honda pair Pedrosa and Marc Marquez, and Tech 3 Yamaha's Cal Crutchlow formed the top five in both sessions, several tenths clear of the rest of the pack.
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