Olympic legend Michael Johnson believes Andy Murray's fitness problems are a thing of the past after training the British tennis star this winter.
Johnson has been impressed with Murray's attitude
The former US sprinter put 19-year-old Murray through a gruelling track session in California last month before designing a series of workouts for him.
"Andy has been doing well with the track sessions and working hard in the weights room," Johnson told BBC Sport.
"I'm certain we will see an improvement with Andy this year."
The partnership came about following a conversation between Johnson and Murray's coach, Brad Gilbert.
"Brad is a long-time friend and neighbour of mine," said Johnson, who is the world record holder at 200m and 400m.
Andy's maturity was more than I had expected
"I just happened to be at his house one day and he asked my advice about working on speed, conditioning and movement.
"That planted a seed in his mind. So we started to talk to Andy and he was very interested in learning how he could improve his speed and condition.
"We decided to go down to the track and then I put together some workouts for he and Brad to do."
Gilbert began working with Murray last July, and one of his first aims was to improve the teenager's strength and fitness.
The Scot's stamina was questioned during his breakthrough season of 2005, when he suffered cramp at Queen's and Wimbledon and vomited during a match at the US Open.
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Murray has worked hard on his fitness since then and Johnson said he was impressed with his overall condition.
"Andy was already in great shape but he wasn't used to track training," Johnson said.
"We started with a session where he had to run 200m within a certain time, have a short rest and then sprint again. We did that over and over.
"At the start it was a shock to his body, but he got more comfortable as he got used to it and it will certainly help his game.
"The aim is for him to go out there and reach a peak in a tennis match and sustain it for a long period of time.
"If he can sustain that top level for much longer, and delay the fatigue, then that is certainly going to be advantageous."
Johnson, 39, said he was very impressed with Murray's approach to training.
"His maturity was more than I had expected," said Johnson.
"Andy is very young, so he doesn't know a lot about training yet. But the advantage he has is that he wants to learn. He's full of questions, which is great.
Brad (Gilbert) understands from his own career that you've got to go out there and work your butt off in order to reap rewards
"When we talked about doing a workout at the track, he wanted to know exactly how it was going to help him.
"He asked me, 'How is this going to help me and is it going to hurt me in any way?' That's great because a lot of athletes go out there and train for hours without knowing exactly what they're going to gain from it.
"That's never the case with Andy, because he's always analysing what he's doing and why he's doing it.
"And he's bought into the fact that Brad is very much into conditioning and hard work.
"Brad understands from his own career that you've got to go out there and work your butt off in order to reap rewards."
Murray is currently playing in the Kooyong Classic ahead of next week's Australian Open, where he will be seeded for the first time.
The Scot is ranked at an all-time high of 15 in the world and will have the same seeding in Melbourne.
Johnson, who runs a company helping athletes with their training, is keen to work with Murray again in the future.
"If Andy needs help, he knows he can give me a call," he said.
"I'll certainly be talking to Brad from time to time. I spoke to him the other day in Australia and things seemed to be going well.
"Brad's always looking for advice and ways to do things better."