Aleix Espargaro ended the first day of the Austin MotoGP in fourth place on the Suzuki GSX-RR but the Spaniard was less than satisfied overall after experiencing more rear-end chatter problems.

Espargaro clocked a time of 2m 05.564s on his final lap in FP2 on the drying track in Texas, but says he still has a 'huge' amount of work to get through with the Suzuki, with the chatter issue in particular requiring further probing.

"I'm happy for the final position; like in Qatar I finished the early sessions with a virtual position near the top which is good, but overall this has not been a fully satisfying day," said Espargaro, whose best lap was 0.729s behind Honda's Marc Marquez.

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"This morning we had to start from zero with the rain since we have no experience in the wet. I feel the bike is performing well, but we still have a huge amount of work to do for the final set-up and on my own I still have a lot to learn about how the bike behaves and reacts.

"In dry conditions we have much more experience, but I have been having the same chatter problem on the rear that we had in the Qatar race. We need to further investigate this and fix it," he added.

"We will see how the weather develops tomorrow. Today's fourth place is a good starting point and we can now concentrate on finding the best feeling with the bike."

Class rookie Maverick Vinales was 15th overall, lapping in 2m 07.011s on the GSX-RR, but was also ultimately disappointed with the first day of practice at the Circuit of the Americas.

"Today was pretty difficult. This morning I had my first experience with the bike in the rain and I feel that I still have a lot of work to do to improve my riding style and also give proper feedback to the engineers to improve the machine," said Vinales.

"In dry conditions I tried to exploit the session to my advantage, but I had few laps available and unfortunately the engine map on my GSX-RR was not the best one so I couldn't be as fast as I wanted, but now we can fix it for tomorrow and try to improve more."