Jorge Lorenzo fears that Bridgestone's 2014 MotoGP tyres are going to prove troublesome for Yamaha.
The double world champion was already concerned that Yamaha was struggling to adapt to this year's fuel regulation changes and their impact on electronics.
After the first day of the second pre-season test at Sepang, Lorenzo said the latest Bridgestones were now the biggest headache.
"The new rear tyres, both the soft and the hard, are not working properly on our M1," he said after going fifth quickest.
"We tried many things but for the moment we haven't found any solutions. Tomorrow we are going to try something different with the electronics that worked last year to try to improve a little bit because we are not so good today.
"The tyres don't give me any confidence in the corner entry, in the middle of the corner and on acceleration.
"It seems Bridgestone will not be providing different tyres this year so we have to adapt to the situation."
But team director Massimo Meregalli believes Yamaha is already making progress with the tyres.
"We are still focused on making the new tyre work properly on our bike and we found something interesting compared to the last test that needs to be adjusted," he said.
"I think we are in the right direction to fix this problem. We have two more days so we expect by the end of the last day we will provide a good performance."
Lorenzo's team-mate Valentino Rossi, who was two places behind the Spaniard in seventh on Wednesday, shared Meregalli's confidence.
"We have a problem with the 2014 [tyres] so we have to concentrate to take the maximum benefit from them but already at the end of today it was not so bad," said Rossi.