Lewis Hamilton must be considered as an all-time Formula One great, Fernando Alonso has claimed.

Alonso, a double world champion, endured a tumultuous relationship with Hamilton in the Briton's rookie season at McLaren.

Indeed their frosty relationship in 2007 ultimately led to Alonso's premature exit from the British team after just one season.

But the pair have reconciled in recent years and Alonso spoke in glowing terms of Hamilton as he closes in on a third Formula One crown.

If Hamilton wins here in Austin on Sunday, and Sebastian Vettel, who has been penalised with a 10-place grid penalty after an engine change, finishes third or lower, the Briton will seal this year's title with three races to spare.

"I think he is one already," said Alonso when asked if a third title would see Hamilton become an all-time great in Formula One.

"He is one of the best in the sport and I think there is more to come. He is one we all respect.

"He's had some up and downs but the good thing about Lewis is that when he didn't have the best car he still won some races, maybe not the championship, but winning some races and fighting for the championship. That's something that maybe not everyone did."

Hamilton, a winner at 20 of the last 35 grands prix, is poised to become the first British driver to successfully defend his Formula One crown.

And with his Mercedes team the class of the Formula One field, Hamilton is in the perfect position to win further championships beyond this season.

Indeed Alonso believes Hamilton, who will remain at Mercedes until the end of the 2018 season, could win as many as six titles.

"We saw Lewis winning in the first years, in the second year, and he could have retired with one title if he didn't have Mercedes," said Alonso.

"Maybe he can win six if no one can beat Mercedes in the next years, but that's difficult to say. Rating a driver is much more than the titles."