Nico Rosberg has weighed in on whether Sebastian Vettel has what it takes to recover his form and challenge Lewis Hamilton after mistakes have hampered the second half of his season.

Rosberg beat Hamilton to the title in 2016 when the pair were teammates at Mercedes so he knows a thing or two about what it takes to beat the prolific Brit and he believes psychology may hold the key.

Four-time champion Vettel began the season well but conceded his lead at the top of the standings at the French Grand Prix in June, where he served a five-second penalty after clipping Mercedes' Valtteri Bottas.

In Germany in July, Vettel crashed out after locking up while leading the race - handing victory to Hamilton and falling further behind the British driver.

Later in September during the Italian Grand Prix, the pair collided on the first lap as Hamilton passed the Ferrari, forcing Vettel to work his way through the field to finish fourth.

When asked what it takes to beat Hamilton over a season, Rosberg told ESPN: "It's all about finding marginal gains. For me, I realised that lots of little things can add up to a very big thing, if you get them all working the right way.

"In 2016 I was speaking to a psychologist, sometimes every two hours a day, and making sure that everything I was doing was perfectly focused on improving every little aspect of my performances. Every driver is different so he has to find his own way.

"The battle against Lewis is so tough because he is so tough mentally. If you don't get that right you aren't going to beat him. If you then start making mistakes as well, forget it.

"This season for Vettel has been a little crazy. He should be in the position to win the title right now and that's got to be tough to take for anyone, especially when the mistakes have been your own and not the team's.

"I don't know if this is the best Lewis has ever driven but when he's competing at the level we are seeing right now only a completely flawless performance is going to beat him most weekends and that's tough when you're on the back foot or you're worried about making more mistakes."

Hamilton leads Vettel by 67 points with just four races of the season remaining, the Brit can win his fifth world title at the US Grand Prix this weekend unless the result falls perfectly in the German's favour.