Aryna Sabalenka says biomechanics coach Gavin MacMillan helped her find a way out of "a desperate situation," and hopes the biomechanics expert can do the same with Coco Gauff.

Coco Gauff has has fired one of her coaches on the eve of the U.S. Open and brought aboard the serve doctor who helped No. 1-ranked Aryna Sabalenka work on her issues with that part of the game.

Coco Gauff, the most popular American tennis player, hadn’t been able to book a practice court. So, she and her newly hired coach, biomechanics expert Gavin MacMillan, braved the mist for close to an hour as they worked on trying to fix her serve.