Sebastian Vettel is not willing to say if Charles Leclerc is his best ever teammate in Formula 1.

In less than a full season, 22-year-old Leclerc has managed to challenge quadruple world champion Vettel's number 1 status at Ferrari.

"He's young, he's very fast. I don't think anyone doubts that," Vettel is quoted by Speed Week.

But is he the best teammate Vettel has ever had?

German Vettel, 32, debuted in 2007.

"You can't really compare them," Vettel said, "because you'd have to put Mark, Kimi, Daniel and Vitantonio in the same car at the same time. So it's not fair," he said.

"In terms of pure speed, Leclerc is very fast, but it is still very early for him. It's only his second season, so it's something completely different than Kimi or Mark, who spent a lot of time in Formula 1 and had much more experience," Vettel added.

But he admitted that it's good that Leclerc is doing so well.

"It's good to have a real reference, and especially good at those times when I had a bit of trouble getting the most out of myself and the car.

"And as a team, it's good to have two riders fighting eye to eye on the track."