Dani Pedrosa put the new Honda RC213V at the top of the timesheets during its first grand prix track session at Laguna Seca on Friday morning.
After two runs with his 'standard' bike, Pedrosa swapped to the new version to set the fastest time, less than one tenth off the circuit record.
But the Repsol Honda rider is yet to decide if he will race with the early 2013 prototype - which he previously rode at the post-race Mugello test - and will continue back-to-back comparisons against the 2012 version this afternoon.
"We had a pretty good start of the weekend. The times has been fast from the beginning and we got into the pace quickly," said Pedrosa who finished the session with a slim 0.074s advantage over Yamaha's title leader Jorge Lorenzo.
"I did my first two runs with my current bike and the last one with the new one and the feelings are quite similar to Mugello, which is positive.
"I'll test both bikes again this afternoon to have more data to judge with higher temperatures and hard tyres and we'll see how it goes.
"This is a demanding track physically, it's small but technical and the bike moves a lot. I think that with the new engine the bike is more stable, smoother due to the engine brake.
"I did my best lap with the new bike, even though we didn't work on the set up yet, so let's see if we can improve it a little more and decide on the best option for the race on Sunday."
Pedrosa is seeking to close a 19-point gap to Lorenzo in the final event before the MotoGP summer break - and the Spaniards dominated the top spot for most of FP1.