In 2015 and 2016 Jenson Button was partnered at McLaren by Fernando Alonso, a driver held in similar high regard to current championship leader Lewis Hamilton throughout F1.
And in his new autobiography, Life at the Limit, the former world champion has compared the two and revealed he thinks the Spaniard is one of the toughest competitors F1 has ever seen.
Button recognises that Hamilton is extremely fast but believes Alonso was more consistent across qualifying and the race over a weekend.
"I'd say that over the years Fernando had been one of - if not the - toughest competitors I'd faced, both as a team-mate and a rival at other teams," Button wrote, as per BBC Sport.
"Lewis was unbelievably quick and could pull a lap out of the bag just like that; him and Ayrton Senna were the two quickest guys over one lap, maybe ever.
"But Fernando was the more rounded driver. I'd know, even if I out-qualified him, that he'd still be tough to beat in a race.
"He's nice on the outside, really affable and approachable, but beneath that he's a very, very tough competitor who'll do anything to beat you."
The pair of them were both unsuccessful in their partnership throughout their tenure at McLaren, with constant engine failures regularly impeding their ability to compete.
Since then, McLaren have decided to split with Honda from next year and Alonso will now be hoping the British manufacturers can start challenging closer to the front of the grid - should he decide to stay on.
Elsewhere, Button also talked about his "weird" relationship with Hamilton; how the two didn't always see eye to eye and often conversations would be awkward.
Button described three-time world champion Hamilton as "one of the greats", adding: "Of everybody on the grid, he's the guy who really has that 'gift'."
But he said their personal relationship was marked by tension throughout their three years as McLaren team-mates from 2010-12.