Repsol Honda riders Dani Pedrosa, Casey Stoner and Andrea Dovizioso occupied second, third and fourth places on the Friday practice timesheets for the US MotoGP - just behind the Yamaha of Jorge Lorenzo.
Lorenzo jumped ahead of 2011 race winner Pedrosa and Stoner during the afternoon session, which finished with last Sunday's winner Pedrosa just 0.141sec behind his fellow Spaniard.
"The second session in the afternoon was pretty interesting, we made some changes on the bike and tried the race tyre, looking to set up the bike to absorb the bumps of which there are many in this track and they especially affect the front," said Pedrosa.
"The grip was a little poor today and it was also very windy out there, so we haven't yet found the perfect ride. In the morning we tried the softer compound and it had a little more grip, and in the afternoon, with the race tyre, it was more difficult to find grip, but perhaps the track conditions were not perfect, so we are hoping to keep improving tomorrow".
Title leader Stoner, fastest in the cooler morning session, took until his final lap to improve in the afternoon, when he finished 0.175sec from the top.
"We've been trying all day to get a slightly different balance with the bike to get the tyres working differently but we're struggling to get the feeling that we want and that we've had in the past," admitted Stoner, now a reduced 15 points ahead of Lorenzo.
"We've changed the set up quite substantially, with pivot positions and trying to get the rear to work with a little less temperature in the tyre, which we've accomplished but still haven't got the grip that we want and we're creating too much wheel spin.
"Also, we're struggling to find a good feeling in the front and just not totally comfortable with the bike in general so we have a some work to do tonight and will continue in the morning to get prepared for qualifying".
There was once again a tangible gap between Dovizioso and his team-mates, which the Italian believes he can close on Saturday, having been almost 0.5sec from Lorenzo.
“This afternoon we did a comparison between two different set ups and the feeling improved," said Dovi. "We still have a gap of four tenths from the first rider, but we lapped more consistently.
"We need to keep on working to improve the pace and the feeling with the front but in general I'm confident that tomorrow we will be able to make another step forward. Compared to the first free practice, lap times came easier this afternoon and tomorrow we will continue working in this direction”.