Valentino Rossi made the biggest lap time gain of any manufacturer rider during day two of the Austin test, but was still left fifth and last of the full time riders, 1.1s behind pace setting Honda rookie Marc Marquez.
Rossi improved by 1.5s during day two of MotoGP's unofficial debut at the Texan circuit - halving the previous 1.2s deficit to team-mate Jorge Lorenzo in the process - but confirmed there was plenty of room for further improvement ahead of April's debut race.
"We improved the lap time a lot from yesterday. We are closer, but there remains one second to the top and there are still some problems," Rossi told the official MotoGP website.
"We will continue our work, continue our development and I will continue to try to arrive at the maximum of the bike.
"It's okay. It was very important to come here because the track is so difficult. So it was good work ahead of the grand prix weekend, but we hope to do better in the grand prix.
"I have problems stopping the bike. I'm not very good in hard braking. Also we have a bit too much of a wheelie issue in acceleration from the hairpin."
With Yamaha opting to miss day three of testing, Rossi will next be on track - alongside the full 2013 rider line-up - during the third and final official test, at Jerez from March 23-25.
Rossi is returning to Yamaha, the manufacturer he won four of his seven MotoGP titles with, after two disappointing seasons at Ducati.