Max Verstappen says Red Bull still have issues to solve if they are to beat Ferrari and Charles Leclerc to Formula One titles this year.
Verstappen is the favourite to defend his F1 championship after winning the Emilia Romagna and Miami GPs, outpacing Leclerc in both races.
However, the Dutchman is concerned about reliability issues after his car suffered a hydraulic leak in Miami on Friday which followed DNFs in Bahrain and Australia. Verstappen's teammate Sergio Perez also had engine issues during the race in Miami.
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Verstappen says he and the Red Bull team are working hard to rectify their problems, but admits there are concerns.
"We're still having a few issues we have to solve," he said.
"We are quick, but my Friday was terrible... which is not great if you want to have a good weekend.
"And also Checo had a few issues in the race, so we have to be on top of that. But clearly there is a lot of potential, we just need to make sure it's reliable.
"We have to make sure these things don't happen... of course, we are working on them flat out."
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Red Bull boss Christian Horner said even with Perez's issues, the team are heading in the right direction.
"Of course reliability is going to be an issue," he said.
"We had a sensor issue on Checo's [Perez] engine and the guys did well to move them around but he lost about 30 horsepower with that. He was losing half a second a lap.
"We saw in Imola how quickly things can change around and we have some interesting races coming up and the car is running well.
"We have some developments coming later in the summer which will help, we need to drop some weight but generally, I think we are on a good trajectory."
The Spanish Grand Prix gets underway on May 22.
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