Jorge Lorenzo said tyre graining masked genuine progress in his adaptation to the Ducati MotoGP bike at Austin.
The ex-Yamaha rider kicked off his Ducati career with an uncompetitive 11th in Qatar and a first-lap crash in Argentina.
He was ninth in America on Sunday, three places off team-mate Andrea Dovizioso and just behind the satellite Pramac Ducati of Danilo Petrucci, having slipped back from seventh place over the final three laps.
But having come in 20 seconds adrift of podium finisher Dovizioso at Losail last month, Lorenzo said running close to his team-mate at Austin and crossing the line just under four seconds behind even with tyre trouble showed he was gaining ground.
"Until I had the front tyre [problem] I was even getting closer to Dovizioso," Lorenzo suggested.
"My position without a lot of graining in the front was seventh, just behind Dovizioso.
"But when I started having graining on the right side of the tyre, probably more than the rest of my competitors, I needed to slow down five or seven tenths [per lap].
"That's where [Andrea] Iannone caught me and also he passed me quite aggressively so I had to go out of the track in sector one and that's why I lost two seconds on that lap.
"And it's when Petrucci came to me and also passed me in the sectors where I lost a lot for the graining in the front."
He said his riding style was prone to creating tyre problems at Austin's Circuit of the Americas.
"Also with the Yamaha I had graining in some years here," he said.
"Especially the triple corner to the right creates this type of graining for a lot of riders here - a little bit more for my riding style, I lean a little bit more on the throttle.
"But this only happens in this type of track. The other ones, the Ducati works very well for the front tyre, we shouldn't have these types of problems."
Lorenzo's up and down weekend included sixth in first practice and on the grid, but in between he had to go through Q1 having only managed 11th - 1.9s off the pace - on Saturday morning.
In Argentina Ducati discovered an error with Lorenzo's seating position that he reckoned had "thrown away" much of pre-season testing.
He said feeling "much more comfortable" on the bike was the main reason why he was optimistic even though results remained low-key.
"This is much more my natural position on the bike and a little bit better even than Argentina, as I could demonstrate in the race," he said.
"The important thing is to feel comfortable on the bike, to feel natural.
"And to be able to push and enter the corners with confidence and positivity.
"This race was not perfect but we are doing it, and I stayed behind Andrea who has been in the team five years so he knows the bike perfectly."