Maverick Vinales credited a new bike set-up for him setting the fastest time in Friday's practice sessions ahead of the Andalusia Grand Prix.
After finishing second in the season-opening race in Jerez on Sunday, Vinales was the pacesetter as the teams returned to the track for the second straight weekend.
The Monster Energy Yamaha rider produced the fastest lap of the day at one minute, 37.063 seconds in FP1, while the winner of the Spanish Grand Prix Fabio Quartararo was down in 13th.
Vinales did not have as much joy following a tweak for FP2, during which LCR Honda rider Takaaki Nakagami topped the timesheets.
"I felt really good on the bike. We tried a new set-up, which is an improvement, and this is the most important thing. Lap by lap and practice by practice I'm feeling better," said Vinales.
"We need to keep working in this way, because I think that's the way to bring the bike to the top.
"This afternoon I was working on the hard and the medium tyre. We need to understand how all the tyre specs work for us, because it will be really hot on Sunday, so we need to know what we can do. But the feeling is great with any tyre.
"We also worked a lot on the front tyre, and the feeling is good – better than it was last week.
"For sure, on Sunday we will have another tyre discussion, but in any case, I feel comfortable with the bike. I think we made an improvement compared to last week."
After retiring from the Spanish Grand Prix in a disappointing start to the season, Valentino Rossi was second quickest – his best lap was 0.142secs slower than team-mate Vinales.
Rossi said: "It's a positive Friday, because we changed something in the setting of the bike, and I feel better. I feel more comfortable, I can ride in a better way, and I improved my pace.
"As always, tomorrow morning will be crucial. It will be very important to find the right spot to make the right lap time to stay in the top 10. We'll have to try.
"I feel better with the bike compared to last week. We have to work because in some corners I'm not fantastic. But anyway, it was a positive day."
Reigning MotoGP champion Marc Marquez did not take part in practice on Friday but is set to feature on Saturday, despite suffering a fractured humerus last weekend.