World champion Sebastian Vettel has improved to the point that he has no weaknesses, according to Red Bull design chief Adrian Newey.
Vettel became only the fourth driver in history to win four world titles with victory in the Indian Grand Prix on Sunday.
Newey admitted that Vettel's driving has had flaws in previous seasons.
But he said: "You can't make those criticisms any more. It's difficult to see a chink in his armour."
Vettel joins Juan Manuel Fangio, Alain Prost and Michael Schumacher on four titles - and only Fangio and Schumacher in winning four consecutively.
Newey, who has previously worked with Prost, Nigel Mansell, Ayrton Senna and Mika Hakkinen has now been responsible for cars that have won 10 drivers' and 10 constructors' titles, giving him a unique place in history.
Newey said of Vettel: "His driving has gone from very talented but slightly raw at times, to incredibly well-rounded now.
"You could occasionally in 2009 and 2010, for example, criticise him for making slightly ill-judged moves and therefore having accidents.
"You could criticise him for possibly not being able to overtake. I think a lot of people felt that if he didn't start the race from the front and control the race from pole then he was not so good. You can't make those criticisms any more.
"It's down to the fact that he learns all the time."
Critics argue that a lot of Vettel's success is down to the quality of the Red Bull car, and that his main rivals Fernando Alonso and Lewis Hamilton could achieve the same things if they had access to the same equipment.
"It's a completely hypothetical question because Formula 1 is about man and machine," added Newey. "It's just not that simple. So it's an armchair question but it's not a question that has any real meaning."
Newey said it was impossible to say whether Vettel, whose victory in India was the 36th of his career, could go on to beat Schumacher's all-time records of seven titles and 91 victories.
"Sebastian is not going to get worse, that's for sure," continued Newey.
"He's going to continue to develop, although it's difficult to see how he can develop relative to this season, but no doubt he will.
"As we can continue to build competitive cars then hopefully Sebastian can continue to win more championships with us."