Valentino Rossi says Yamaha has a lot of work to do on its 2019 engine if he and Maverick Vinales are to challenge Honda's Marc Marquez or Ducati's Andre Dovizioso.

Seven-time MotoGP champion Rossi is yet to record a win this season and sits 87 points behind championship leader Marc Marquez with five races remaining. He is the highest-placed Yamaha rider, though, and had hoped the 2019 engine would enable him to close the gap in the standings.

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However, the Italian was not impressed with his first test of next season's offering.

"I tried the first edition of the 2019 engine, yes, but I think and I hope it's not the final one, because it's very similar [to the 2018 one]," he said, as quoted by Autosport.

"I hope they continue to work, because the engine for me is a problem for us. They have to make it better.

"Unfortunately in testing we haven't tried many things for next year."