The German, who left the sport in December after beating the Briton to the title and was attending the Japanese Grand Prix as a pundit for Sky Sports television, said Hamilton was one of the best of all time.
“It’s always going to be difficult to beat him. His pure speed is just immense,” Rosberg said. “Fortunately I managed to do it a couple of times.”
Hamilton’s pole was his first at Suzuka, 10th of the season and 71st of his career.
So dominant was the triple champion that he obliterated the previous circuit qualifying lap record set by Michael Schumacher in 2006.
The Briton is 34 points clear of closest rival Sebastian Vettel, of Ferrari, with five races remaining and clear favourite for a fourth crown.
“The run after the summer was really important for me and Lewis has that run at the moment,” said Rosberg, who won in Belgium, Italy and Singapore last year after the summer break to leave Hamilton with a mountain to climb.
Hamilton won the last four races but it was not enough. This year he came back from the August break by also winning in Belgium, Italy and Singapore.
“He has the momentum completely on his side and it’s totally against Sebastian at the moment. Everything is going wrong more or less there and now he doesn’t have the car performance here either,” Rosberg added.
The retired champion, whose rivalry with Hamilton started out in a friendly fashion when they were teenage karters but turned almost glacial by the end, said the Briton was also mastering the mental side.
“That’s what Lewis has managed to do for quite a while now… and that’s what’s brought him this huge lead, and why everything is in his favour at the moment to take this thing home all the way to the end of the season,” he said.
Vettel starts alongside Hamilton on the front row at Suzuka on Sunday and will need to seize the initiative with a lightning start at a track where overtaking is otherwise difficult.
Rosberg, last year’s winner in Japan from pole, said he had been loving the battle between the rivals.
“Both are fantastic drivers, both are up there with some of the best of all time. I don’t think it’s going to make any difference who wins this title,” he said.
“I think it’s just been a great season for F1, I really enjoyed watching it on TV, all the battles. Both have their strengths and weaknesses and it’s cool how they have been playing out.”
(Reporting by Alan Baldwin; Editing by John O’Brien)