Alderweireld will come up against Costa tomorrow as Spurs take on Chelsea at Wembley in the FA Cup semi-final.
But he first came across the Spain international when he joined Atletico Madrid back in the summer of 2013 from Ajax.
During their time together at Atletico, Alderweireld marked Costa in training.
Tottenham star Toby Alderweireld claims Chelsea's Diego Costa is the toughest striker he has faced
And the Belgium international claims his time spent training with Costa made him a better defender.
“Was he [Costa] the toughest forward I marked? Yes, he’s a winner,” Alderweireld told the London Evening Standard.
“He is talented, strong, clinical in front of goal and he has a good technique.
Chelsea line-up predicted to face Tottenham Hotspur in the FA Cup semi-final Thu, April 20, 2017
Express Sport predicts who will start for Chelsea in their FA Cup semi-final clash against Tottenham Hotspur at Wembley on Saturday (5.15pm)
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Who will Antonio Conte pick to start for Chelsea against Tottenham?
Diego Costa was team-mates with Toby Alderweireld at Atletico Madrid
"He is one of the best in the world.
“I was at Ajax as a young kid and then I went to Atletico, playing with the big boys.
"It helps so much to train with people like Costa."