Reigning MotoGP champion and early 2013 title leader Jorge Lorenzo was concerned to lose ground to the Honda riders as the Austin circuit improved during Friday practice.
Lorenzo made a strong start in the cold and slippery morning session, finishing second and just 0.253s behind Repsol Honda's Marc Marquez.
But he then slipped back to fourth as the circuit began to unleash grip in the warmer afternoon session, losing out to Dani Pedrosa (Repsol) and Stefan Bradl (LCR) as well as repeat pace setter Marquez.
“In the morning I was surprised to be so close to Marc, but this afternoon when the track improved they improved much more than us,” Lorenzo stated. “We couldn't get the best performance because we still have a lot of problems on the entry to corners. I don't have confidence.”
Lap times are expected to tumble further on day two - Friday's times having been several seconds off those set during a private test in March, when Honda also held the advantage over Yamaha.
“From the outside it looks like the Hondas can turn much more easily," explained Lorenzo. "They can open the throttle and pick the bike up sooner. We stay a lot longer on lean angle with no grip on the rear. Like this it is difficult.
“But I'm positive. Anything is possible and if we can improve the bike we can get much closer tomorrow.”
Team manager Wilco Zeelenberg confirmed the main overnight focus will be on rear grip.
“We're position four so of course we're not so happy; we're struggling a bit to find the correct grip,” said the Dutchman. “We are trying to find some extra grip, to drop the lap times we need more on the back. Hopefully tomorrow we can find the right way to improve and close the gap. It's important to be on the front row and close to the fast lap times.”
Lorenzo's team-mate Valentino Rossi was 0.2s slower in fifth, with satellite Yamaha rider Cal Crutchlow sixth. The only Honda rider to be beaten by the Yamahas was Gresini's Alvaro Bautista, who did not attend the private test and was 0.6s behind Lorenzo.