MotoGP rookie Marc Marquez retained the top spot as lap times tumbled during Wednesday's second day of private testing at Austin, Texas.
Improving track conditions, temperatures and set-up refinements allowed reigning Moto2 champion Marquez to clip 0.766s from his day one best, enough to keep him clear of experienced Repsol Honda team-mate Dani Pedrosa by just 0.123s.
Marquez, who set his best time on lap 39 of 55, spent the day testing some electronic settings and generally building confidence with his RC213V.
"Although it looked like the track had a little less grip, it wasn't long before I could ride at the same level as yesterday,” said the 20-year-old. “I gradually improved with every stint. Having a good pace early was important, because it allowed me to start to try things out.
“For the rest of the day we kept dropping our times and keeping to our strategy: testing things and confirming the results to eliminate any doubt. We did a great job today and the team is really helping me a lot.
“I am learning so much during these days - most of all with the electronics. With this being a new track, we have no base to go on and we have to work a little harder on finding one.”
Third fastest behind the Repsol riders was Yamaha's reigning champion Jorge Lorenzo (+0.498s), followed by LCR Honda's Stefan Bradl (0.787s) and Lorenzo's team-mate Valentino Rossi (+1.107s).
Unlike Yamaha, the Honda riders will return for the third and final day of testing.
This week's test marks the first MotoGP laps at COTA, which will host round two of the 2013 world championship, by the pace setting Honda and Yamaha riders. Ducati elected not to attend the test.