Valentino Rossi was left on the back foot after missing almost half of the second MotoGP free practice session at Aragon with a brake problem.

Rossi admitted he struggled more than he had expected at the undulating Spanish circuit with the Factory Yamaha M1, where he concluded the day in seventh position, 0.922s off the pace set by Repsol Honda's title leader Marc Marquez.

The nine-time world champion had been lying in fifth place in FP1 this morning, but was left frustrated following a number of issues in FP2, including brake and grip trouble.

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"We struggled more than we expected. In the [June] test the track had different conditions and more grip, the temperature was lower and it was better," he said.

"Today we suffered quite a lot, especially this afternoon I had quite a lot of issues - especially one with the brakes and I lost nearly half the practice.

"At the end I used the second bike and made some laps to find the rhythm. We have to improve and go faster but at this moment after some laps the rear tyre starts to slide a lot and the bike becomes difficult to control."

Rossi's team-mate and title contender Jorge Lorenzo was again the top M1 rider, but this time only fourth on the timesheets (+0.418s).

"We didn't expect a start like today," admitted Yamaha team director Massimo Meregalli. "Unfortunately the bike didn't work as it did during the test. The track conditions were completely different.

"We have a lot of work to do to improve the bike, the grip levels on the track are not good, one more day will make it much better. We will analyse the data, compare it with the riders' feedback and come back tomorrow to make another step forward."