British number two Aljaz Bedene says he had to employ a strength and conditioning coach to specifically prepare him for grand slam tennis.
The British number two, who plays world number one Novak Djokovic in the third round of the French Open on Saturday, credits his work with Martin Skinner as the platform for his recent success.
"I was really unfit when I started to play grand slams," said Bedene.
"Whenever I played a fourth set I was just losing them quite easily."
The Slovenia-born player, 26, added: "I remember once at the US Open, I lost 6-0 and I was struggling.