Ferrari have been warned that Max Verstappen is still not at his best this season despite winning the first two races for Red Bull.
Former F1 driver Johnny Herbert has seen positives for Ferrari and Charles Leclerc but is wary of Verstappen having more in the tank.
Verstappen cruised to victory in Bahrain and Saudi Arabia as he aims to win a fourth successive World Drivers' Championship.
His team-mate Sergio Perez finished second in the two races, while Ferrari have had to settle for third place on both occasions.
Ferrari have been warned that Max Verstappen is not yet at his best this season
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Ferrari were the only team other than Red Bull to win a race last season when Carlos Sainz triumphed at the Singapore Grand Prix.
Herbert has seen improvement at the Italian team, especially when it comes to race day, and told RacingNews365 that 'only Charles and Ferrari can challenge' Red Bull.
He said: "The problem is the race situation. (Ferrari) are at least better off than they were before because their tyre deg is definitely under control.
"They can basically push the whole way through a race, so that's good, that's positive.
"It's still going to be down to the classic situation that the updates that come along have to make an impact, to improve the Ferrari, of course, but the impact of getting closer and putting Max and Red Bull under pressure.
"Equally they (Red Bull) are going to be doing very similar things but it's who can do a bigger job.
"Ferrari should be able to do so just because Red Bull is ahead of the game anyway, and ahead of the pack."
However, Herbert's main concern for Ferrari is that Verstappen has not yet reached his top level.
He claims the 26-year-old has not had to stretch himself to his limit and believes he will do so if pushed by Leclerc.
Herbert added: "My only little chink of worry is that I don't think we've seen the best of Max this season anyway because he doesn't need to show it.
"That's the worry, that he still has something in his back pocket where he can go 'Okay, you think you're closer, there's another couple of tenths'.
"I think that's probably the only thing that's going to be a little bit more of a headache for Charles and Ferrari, and for when Carlos comes back, because he (Verstappen) is so comfortable in the car, and they are in total harmony together.
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Max Verstappen has won the first two races of the season for Red Bull
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"I have said this many times over the last couple of years, but it is still a very rare thing to see someone so at ease with his driving in the car, and we still haven't seen the best of Max anyway."
Ahead of next weekend's Australian Grand Prix, Herbert believes Ferrari can cause Red Bull some problems.
But he thinks it is too early for them to compete for a World Championship.
Herbert said: "I know there's been all the chit-chat over the advantage they (Red Bull) have, and all the teams have said 'Well, they've done a better job'.
Charles Leclerc finished third at the Saudi Arabian Grand Prix
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"And that's all it's down to, that they've done a much smarter job than everybody else.
"But there are smart people in Ferrari that I think can challenge.
"Can they challenge for the championship? That's probably the biggest question but I'm not sure they'll be able to do it quick enough to stop Max going on to win another one."
