Reliability issues for Ferrari have effectively handed the title to Mercedes driver Lewis Hamilton, says Jenson Button.

Hamilton has opened up a 59-point advantage at the top of the standings having won four of the five grands prix since the mid-season break, while Vettel's challenge has slipped as a result of engine problems.

After crashing out in Singapore, the German had to fight from the back of the grid for fourth in Malaysia and was unable to finish in Japan.

Hamilton can consequently clinch a fourth world championship if he wins this weekend's United States Grand Prix and Vettel finishes sixth or lower.

Button believes his former McLaren team-mate would be a worthy victor but laments the impact Ferrari's troubles have had on an enthralling title race.

"In terms of the fight this year, it's good that we have another team that's challenging Mercedes and Lewis. I think that keeps people interested," Button told Omnisport at an event to promote his new book.

"I've watched every race. I didn't think I would, but I have, because I love that fight between two manufacturers, two great drivers, but I think that's what people will turn on to see.

"It's a shame that Ferrari have had the issues they've had the last couple of races and Sebastian obviously with the incident in Singapore because they've been quick, they've just made mistakes and sort of handed it more to Mercedes and Lewis.

"But if Lewis does win the championship, which I think he will, it's very well deserved. I think the last two years he's been driving very well, very confident it seems in his ability.

"Last year he was just really unlucky with reliability issues but this year it's just run smoothly and perfectly for him. I hope he clinches it."

Button has admitted to concerns the title was going to get away with him in 2009, when he failed to guide his Brawn GP to the top step of the podium after triumphing in six of the season's opening seven grands prix.

Asked if Hamilton could have similar worries as the season enters its closing stretch, he said: "No, because his car is still the quickest.

"We were so far off the pace by the end of the season. We didn't have the funding that Mercedes has. We spent very little during the year so we didn't develop the car.

"It's dangerous to stand still in Formula One because everyone's improving the whole time. For us it was very difficult at the end of the year.

"For him they have the best car, or maybe second-best car, so he's just got to be consistent and finish."

Jenson Button's new book 'Life to the Limit: My Autobiography' from Blink Publishing is available now.