REVEALED: Coco Gauff's coach Brad Gilbert gave tennis lessons to Zendaya and her Challengers co-stars Josh O'Connor and Mike Faist: 'They put it in the hard yards'

Hollywood turned to legendary tennis coach Brad Gilbert to help Zendaya and her Challengers co-stars hone their skills on the court.

Gilbert is one of the most celebrated minds in tennis, having worked with the likes of Andre Agassi, Andy Roddick, Andy Murray, and now Coco Gauff. 

The movie, which hit America's big screens this week, follows a tennis champion (Mike Faist) as he attempts a comeback with the help of his wife, a prodigy-turned-coach (Zendaya). But one of the opponents standing in his way is his former best friend and his wife's former lover (Josh O'Connor).

Gilbert, who helped Gauff win a maiden Grand Slam at last year's US Open, was named tennis consultant for the movie.

Over three months of training in Boston, he put the actors through their paces for two hours every morning.

His job was to 'make sure everything was authentic as possible... all of the players, the opponents, the linesmen and ball people.' 

That included working with the stars and tennis pro body doubles. Faist, for example, had to eat around 10,000 calories a day to have the shape required for his character. Gilbert's wife Kim also 'sent them all a ton of videos to help them understand'.

Faist’s character Art was supposed to be 'this classic player with a one-handed backhand, a little bit patterned after a Pete Sampras,' while Zendaya played Tashi, a 'killer' on the court, and O’Connor’s Patrick 'was more of this free wheeler.'    

'They put it in the hard yards,' Gilbert said, per People. 'The focus that actors and tennis players have is similar.'

Faist, Gilbert revealed, 'was the only one that had played, in high school', while Zendaya had only played a handful of times as a child. 'Josh literally had never played,' the coach added.

Practice began at 7am every morning. 'Then all afternoon they had to work on their acting... so, we had long days to get ready just to start filming,' Gilbert added.

'In tennis, nothing is choreographed. It's in real time'. But while making a movie, even short rallies needed to be rehearsed '50 times'.

'I had an actor on each different court practicing their routine, what they needed to do, so learning (their) part for each one of those points,' he said.

During one rally, O'Connor was tasked with hitting a tweener - a between-his-legs shot.

'We practiced this so freaking much, and that was the toughest shot for Josh to be able to do,' Gilbert remembered. 'A one-in-a-million shot.' He managed it in practice. 'I could not believe we didn't have it on film,' Gilbert added.