Alonso thinks that the true test of Vettel's ability will come later on, when his car is not as dominant.
"Sebastian's been dominating the last couple of years," the Spaniard told reporters. "He's a fantastic driver and has a fantastic car.
"He's been close to perfection all these years, winning world championships. It's true he has an advantage in car performance for this year so we'll see how good he is later in his career.
"We also saw [Lewis] Hamilton almost winning the Championship in his first year, win it in the second and then he didn't win any more because sometimes you have the car to win, sometimes not."
The Japanese Grand Prix takes place at 7am on Sunday morning.