The Honda rider, who graduates to the premier class after winning last year's Moto2 title, set the pace on all three days at the US track.
Marquez had also been near the front in both group tests at Sepang, prompting Nicky Hayden to predict that he could change the face of grand prix racing.
Marquez however cautioned that the Austin test was not wholly representative and that he expected to be at more of a disadvantage on existing grand prix tracks on which his rivals have previous MotoGP experience.
"These three days were a little fictitious and the reality will be that everyone will be closer together," Marquez said.
"Of course, this result was better than I was expecting. [But] we have to look at the reality and not expect to go so well.
"In the end all world championship circuits are places where everyone else has ridden for years with these bikes. They have experience and a solid foundation.
"For me, this will all be new."
Marquez did admit to being pleased with both his ability to adapt to the Austin circuit and his increasing familiarity with Honda's RC213V.
"At a new circuit like this I adapted well, and that is a boost," he said.
"In Austin I already understood the bike a little better. With no base setting you have to work a lot more and, although it is tougher for the rider, in the end you learn more from that.
"I started to enjoy myself, I am a little looser [on the bike]. There are still moments where I stiffen up, but riding loose allows me to be more consistent.
"The truth is that at this more recent test I picked up a lot more. We planned it well."
- Sports & Recreation
- Nicky Hayden