Roy Keane had no doubts when asked to name his toughest opponent after refusing to say Patrick Vieira

Roy Keane has named his toughest-ever opponent and he refused to pick his former bitter rival, Patrick Viera.

The former Manchester United captain is widely regarded as one of the toughest competitors the sport has seen in the modern era, with many well-documented bust-ups throughout his career.

Keane played in a time of strong, tough-tackling central midfielders, and often came up against the world's best during his time at Old Trafford.

Speaking in an interview with ITV earlier this month, Keane revealed which opponent gave him the hardest time and jokingly opted against Vieira.

"I'm not going to say Vieira, no, truthfully I'd say Zidane would probably be the toughest opponent," Keane said.

"He's a big strong boy, technically brilliant, very aggressive, scored on the big occasions so Zidane would be the toughest player I'd have played against," he explained.

Keane came up against Zidane on four occasions in the Champions League, with the Frenchman winning twice and Keane also winning twice including a 3-2 win at Juventus in United's 1999 treble campaign.

Roy Keane and Zinedine Zidane following Ireland vs France- (credit: Getty)

Despite snubbing Viera, Keane has made it very clear since his retirement how much respect he has for the former Arsenal man.

Speaking when Vieira appeared on The Overlap last year, Keane stated that the rivalry helped get the best out of him.

"I feel when players come along like Patrick they will bring the best out of you, that's what the game is all about. You'd have to bring your best game, your A-game," he explained.

"Who won more battles? Obviously, it's very difficult, Patrick would have had his good days and I had some good days."

"I think the clashes were good, I certainly enjoyed them.

"I think the battles I had with [Viera] were honest, there was nothing sneaky about it, you hit me and I hit you and we get on with it," he added.