Marquez currently leads the MotoGP world championship in his first year, holding a 30-point advantage over Dani Pedrosa with six rounds to go.
Rossi finished second as a 500cc world championship rookie in 2000, but only won twice and ended the campaign 49 points adrift of Kenny Roberts Jr.
The Italian, who then went on to win the next five consecutive titles, said he outpsyched himself in his debut season and Marquez is showing no such weakness.
"What Marquez is doing this year is very impressive," said Rossi when asked by AUTOSPORT how Marquez's progress compared to other highly-rated debutants over the years.
"For me, more than anything he has the mind. He started this year to win the championship.
"In 2000 when I was a rookie I didn't do that; I thought in my first year it is impossible. Maybe for that reason I didn't win.
"But Marquez arrived with the mindset to win in his first season.
"I have never seen a rookie like him."
Although the Honda is now regarded as a superior bike to Rossi and Jorge Lorenzo's Yamaha, Rossi said that did not stop Marquez's pace being breathtaking.
He cited Marquez's narrow victory over Lorenzo in their lap-record-shattering Silverstone pole fight as an example of how the Spaniard is raising the bar.
"Marc is unbelievably fast," said Rossi.
"Jorge was impressive, but Marquez, as a rookie riding at Silverstone in MotoGP for the first time, was something more."
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