"The last three or four years, I think I always rode the M1 (bike) on my birthday," said Rossi, whose contract runs out at the end of this year but has shown no signs of wanting to retire.
"From one point of view it's a bit difficult, because you have to work, but from another point of view this is the best way to spend my birthday."
LCR Honda's British rider Cal Crutchlow was fastest in the heat, despite a crash, with Suzuki's Spanish rider Alex Rins second and reigning world champion Marc Marquez third on a factory Honda.
"It seemed quite fast to me when I lapped it on a scooter yesterday, but today riding my bike, I realized it was slower than I was expecting, with many second or third-gear corners," Marquez said of the layout.
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"Still, there are some hard acceleration and braking points, and it will probably be challenging to manage tyre life, so we’ll work on that as well."
Thailand hosts the 15th round of the season in October. The first race is in Qatar on March 18.
(Reporting by Alan Baldwin in London, editing by Toby Davis)
