Hamilton will become only the third man to clinch a fifth world title if he wins Sunday's US Grand Prix and Sebastian Vettel is lower than second.

Alonso named Hamilton with Michael Schumacher, Juan Manuel Fangio, Ayrton Senna and Alain Prost as his top five.

Hamilton winning a fifth title would be a "great achievement", added two-time champion Alonso.

"It is difficult to compare different times and different ways to win those championships," said the Spaniard, who tied on points with Hamilton when they were McLaren team-mates in 2007, with both men finishing one point behind title winner Kimi Raikkonen.

"Lewis probably winning five now, same as Fangio, it is a great achievement, and if one had to do it in our generation, I am happy it is him.

"He showed the talent and commitment. When the car was dominating he won and when it wasn't he still put in performances to show his talent. That is difficult to see these days."

Hamilton is the only driver to win a race in every year he competed in F1.

Alonso said: "He showed the talent from day one and fighting for the championship in his rookie year, winning in 2008.

"I am happy for him because he showed the talent from day one, he was able to win races when the car was able to win it, but also able to win when the car was not on top of the form, he still won some races, it is impressive."

If Sebastian Vettel does not finish in the points then Lewis Hamilton only needs to come sixth to become the world champion

Hamilton said he was not approaching this weekend in Austin, Texas, thinking about winning the title.

"It just feels like another race weekend that we want to win," he said. "It is a simple goal to arrive and try to do the same thing, the weather is up and down and the goal is to have the same performance as in previous races."

Hamilton is 67 points clear of Vettel - with only 100 available over the remaining four races - after winning six of the past seven races.

"We expect Ferrari to punch back hard here this weekend," he said. "We can't be relaxed. We have to come here and raise the bar.

"I don't need more pressure, I am just trying to focus on how I have driven all year long. It is going to be a tricky weekend. Nothing is given."

As for his achievement in potentially matching Fangio's tally of five titles - with only Schumacher ahead on seven - Hamilton said: "Fangio is like the godfather of the sport for us drivers. It is crazy to think I am embarking on a similar number of championships."

Vettel, who has not finished in the top two in the past four races, said: "It is pretty clear. We need to be quick enough to win the races and then it depends on what the other side is doing. Focus on that.

"The last couple of races we have not had the speed so we have some ideas floating around and hopefully we get back. Let's see what the next four races bring. We hope to be back to the form we showed earlier.

"I always try to win. You have to be aggressive to win. You need to find the right balance. You never try to do something silly not to finish because that way you can't win.

"I know what to do. We know we need to take a bit more risk here or there because we are not in a position to manage a gap in the championship because we do not have a gap."

How to follow the United States Grand Prix on BBC Sport

BBC Sport has live coverage of all the season's races on BBC Radio 5 live and BBC Radio 5 live sports extra, plus live online commentary on the BBC Sport website and mobile app - including audience interaction, expert analysis, debate, voting, features, interviews and video content.

All times BST and are subject to change at short notice.