Jorge Lorenzo worried by Yamaha's tyre degradation in Spanish Grand Prix practice

Lorenzo leads the championship after passing Honda's Casey Stoner for the lead in the latter stages of this season's opening round in Qatar, but he has concerns in Spain after he and team-mate Ben Spies suffered from tyre problems in the mixed conditions of practice.

Lorenzo says the team needs to work on the tyre issues before the race on Sunday.

"In these conditions we are not so good because the middle of the tyre is dropping a lot, after eight or 10 laps we don't have any more grip so we have to slow the pace," he said.

Lorenzo said he may struggle through the weekend if the conditions are changeable again.

"If it rains with a lot of water then that's one story but if it's like today we are not the fastest so we need to gain some grip on the rear."

Spies, who took second place in Qatar, says that he was able to make some changes to improve his bike but was also hampered by tyre wear.

"We started with a wet track and we had to fiddle round with some tyre pressures for a while. We got that sorted and made some changes to the bike to the point where I was comfortable. The tyres had got quite bad by that point," said Spies.

Team director Massimo Meregalli said Spies had found a better balance with his bike than team leader Lorenzo.

"We found a balance that Ben liked and he seems quite satisfied with the set-up so we'll see for tomorrow. On Jorge's side we are still hunting for some more grip from the rear in the difficult condition but we should make a step tomorrow."