Hamilton and Vettel would have 'hard time' in Formula E

Di Grassi raced against Hamilton and Vettel in Formula 3, finishing third in Hamilton's European title-winning 2005 season, when Vettel was fifth.

The Brazilian spent the next season with GP2 backmarker Durango while Hamilton clinched a second major junior title in a row, and shared the F1 grid with both drivers in 2010 when he drove for the new Virgin Racing squad.

Speaking to CNN ahead of this weekend's FE race in Paris, di Grassi said: "They would have a hard time coming to Formula E.

"They are fantastic drivers, they are the most talented drivers of our generation and maybe, I would say, almost our lifetime, but if they come to Formula E they would find it hard.

"They will have to learn the tracks, because all the tracks are brand-new, they are all city tracks, you cannot practise on the tracks.

"I think I will invite them to come to race, even if it's just one race, and see how it is.

"I'm sure they would be tough competition, but they would have a hard time."

Di Grassi leads the FE standings by point ahead of Sebastien Buemi with five races remaining.

He later reiterated the comments to Autosport.

"I raced with them all my life," he added. "That generation, which was mine, was extremely talented.

"Nico Rosberg and Robert Kubica, Kazuki Nakajima, Vitaly Petrov, Sebastien Buemi and Timo Glock - all those guys were really highly talented.

"But it's a completely different championship and the level is very high.

"They will succeed at a certain point, but to be able to sit in the car, which is very different, and be fast straight away and understand the energy concept is very difficult."