Having struggled for grip in the slippery morning practice session on the new Indianapolis asphalt, tyre wear became the priority for many MotoGP riders by the end of the afternoon.
Mapfre Aspar's Hector Barbera explained: "We went out with a soft front tyre at the start of the afternoon and the carcass was moving around quite a lot, so we switched to the hard and my feeling improved a lot.
"It looks to be the better option, but the track surface is like sandpaper and with five laps there is no rubber left on the tyre.
"Overall it has been a positive first day but we have work to do tomorrow, starting with the tyre pressure to see if we can get it working better and lasting longer."
Barbera was eighth and 13th fastest in the two sessions.