MotoGP champion Jorge Lorenzo lost out on a Friday practice sweep at his home Catalunya round by just 0.049s to Yamaha team-mate Valentino Rossi.
Rossi moved to the top after a late change to the softer rear tyre, with which Lorenzo had set his best lap of the day early in the afternoon session.
Honda title leader Dani Pedrosa completed a top three covered by just 0.148s, but the roasting temperatures mean that prolonging tyre life is being seen as the crucial factor for Sunday's race.
“We don't focus so much on the classification because not all the riders kept on the same tyres during practice, a new tyre can give you half a second or more,” said Lorenzo.
“Our bike is good enough to stay consistently in the first three positions, but we still a little uncomfortable. The temperature is hot and the performance every lap is getting slowly worse.
“We stayed with the same tyres for the whole practice to understand why they are dropping in this weather so we could solve this problem that I think all the riders are having.
“Valentino wanted to try the hard rear so we decided to wait to hear their comments about the hard rear tyre. But finally we didn't test the hard one. They said it is not better than the soft one.”