Vettel will bow out of F1 at the end of the season, enjoying great success throughout his F1 career, which started in 2007.
The German won four consecutive championships with Red Bull from 2010 to 2013, defeating Alonso to win two of them.
He went up against Hamilton in 2017 and 2018 with Ferrari, losing out to the Mercedes driver in both seasons.
Despite a dry spell in terms of victories in the last three years, Vettel remains third overall behind Hamilton and Michael Schumacher in the all-time winners list, while he remains third in the all-time podium scorers.
In an interview on Aston Martin’s official website, Vettel was asked who was the toughest driver he faced during his career.
“Lewis [Hamilton], especially when I was at Ferrari,” Vettel said. “Lewis has always been right up there.
“Before that, it was probably Fernando [Alonso] when he was at Ferrari.”
Vettel secured his first title at Alonso’s expense back in 2010.
Going into the season-ending Abu Dhabi Grand Prix, Vettel was 15 points behind the Spaniard, meaning Alonso needed just a P4 to secure his third championship.
Vettel dominated the race, however, Alonso only managed to finish seventh after a strategy blunder from Ferrari dropped him behind traffic, most notably Vitaly Petrov, who he couldn’t overtake.
In 2012, Vettel was in the driving seat after finding his form in the second half of the season.
Alonso should have never been in title contention given how poor the Ferrari was for much of the year.
Things looked to be going Alonso’s way when Vettel spun on the opening lap, sustaining significant damage to his Red Bull.
After a crazy race in changeable conditions, Vettel recovered to finish sixth, ultimately taking his third title by just three points.
Ironically, Alonso will be the man to replace Vettel for F1 2023.
Vettel added: “Fernando doesn't need any advice. I'm not sure he's taking any advice, but he doesn't need it anyway. He's been around for so long and seen so much - he'll be fine.”