Season points leader Maverick Vinales set the practice pace Friday at the Moto Grand Prix of the Americas with Spanish compatriot Marc Marquez, the four-time defending champion, settling for second-best.
Yamaha rider Vinales, who won the first two races at Qatar and Argentina, completed the fastest lap over the 5.5km, 20-turn Circuit of the Americas at Austin, Texas, in 2mins 4.923secs with Honda's Marquez .577 of a second back in their first runs over the course.
"It's my first time here with the Yamaha M1 and I felt great," Vinales said. "The bike is working so good. We still have to work on the electronics but we are happy with the set-up. The bike is working at 100 percent.
"Of course we still have to work hard, especially in preparation for the last laps of the race, but I'm happy and confident. I'm sure that we can do a really good weekend."
Vinales won at Texas in the 2014 Moto2 race and just missed the GP podium last year riding for Suzuki.
Marquez, the reigning world champion, has won all four prior Moto GP events contested at the circuit from the pole. He tested different set-ups on a track he found much bumpier than in past races.
"I felt quite good on the bike," Marquez said. "We did two long runs with the two bikes set up differently, one with a setup similar to what we used in Argentina and another very different in order to understand which works better for this track this weekend.
"Overall, the feeling wasn't so bad. I think we did a good job on used tires and we found a good pace, though we have a margin for improvement, especially on the electronic side. The circuit has changed a lot from last year, as it's now very bumpy... we'll work more on the bike's setup, also looking to cope with the bumps better."
Vinales leads the standings with 50 points to 36 for Italian teammate Valentino Rossi, who started third last year but crashed out on the second lap.
Rossi was eighth in the opening practice run, 1.369 seconds off his teammate's pace.
"COTA is always a difficult track for us, but it's also very interesting," Rossi said.
"The first session this morning did not go very well but it was not bad either. We still have to work a bit and find the right balance of the bike. We need to improve in the corner entry and make the tires work better."