When Andy Murray published a video of a recent gym session during which he did a handstand, fellow tennis player Nick Kyrgios responded: "I see you've lost your marbles without me."
It seemed an unorthodox way to prepare for the new season, as the Scot continues his long road back from hip surgery last January.
But the coach making Murray leap and bound all over his Philadelphia gym has become a go-to man for sportspeople returning from serious injury.
Bill Knowles was one of the first people to text Danny Welbeck after he broke his ankle last month, having previously worked with the England footballer.