The Dutchman is on a run of six successive points-scoring finishes, a record for a Toro Rosso driver, with fourth places at the Hungaroring and in Austin the highlights.

He has caught the eye with his overtaking, including a pass around the outside of Sauber's Felipe Nasr, at Blanchimont at Spa, although he has sometimes come under fire from more experienced drivers for incidents such as his clash with Romain Grosjean in Monaco.

"Max has been a surprise because he's been very aggressive, sometimes too aggressive," said Vettel, who won the Race of Champions at London's Olympic Stadium on Saturday.

"But in the end, he has done a very good season so far, he has had two fourth places.

"He is one to be up there in the future. With his age, he has a lot of time.

"I'm sure he'll make us worried, the older generation, which I'm part of now."

Vettel has also been impressed with Verstappen's team-mate Carlos Sainz Jr this season.

The Spanish rookie has scored points seven times this season but he has been hit with poor reliability in the second-half of the season.

"Carlos and Max have done a very good job," said Vettel. "Those two stood out this season.

"I know them, Carlos in particular, from the past from my time at Red Bull Racing.

"He was helping a lot on the simulator, doing a lot of work for us, so it's good to see them racing."