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Preparing his body for the game enabled Agassi to play with "great relevance" right up to age of 36, says Reyes.
The American's latter playing years were the most fruitful of his career with five of his eight grand slams won after his 29th birthday. He was also world no.1 at the age of 33.
"When you stop and think about it, that's amazing," Reyes says.
Strength was the key to Agassi's enduring success, Reyes believes, who recalls "a rather slender fellow" when they first met in 1989. But together they devised a fitness program tailored to the stresses and movements Agassi experienced on court.
"If we were doing certain running drills, he would say: 'I don't feel that on my legs the way I do on the tennis court.' That was beautiful because then he would leave it to me to come up with the training," Reyes said.
Reyes would also tailor gym work depending on the playing surface -- the high bounce at Roland Garros requiring different strength training to the skiddy low bounce of the All England Club's grass courts.
"I can say, honestly, that he was, if not the strongest player on the tour, one of the strongest. Playing to 36 in this day's game it takes a lot."
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Reyes thinks the next generation of tennis stars will reap the benefits of improving sports science but does not envisage too many current players emulating Agassi, with one notable exception, perhaps.
At the start of the year, a back injury forced Federer out of the Qatar Open in Doha. It was only his second withdrawal during a tournament in his career and prompted speculation that 2012 might be a year of diminishing returns for the great champion.
Ten months on, Federer has a further six titles under his belt, and heads towards November's season-ending ATP World Tour Finals with just his own records as the oldest winner and six-time champion to beat.