Nadal, 31, had to retire from his quarter-final clash with Marin Cilic after suffering a hip flexor injury.

The Spaniard was at the top of his game last year, winning two Grand Slams and finishing as world No 1.

Before 2017 Nadal enjoyed a three-month break - enforced due to a wrist injury - while this year has been different.

He pulled out the ATP Finals with a long-standing knee problem and Rusedski feels he hasn't had time to get his body right.

The pundit told Eurosport: "Last year he had three months to prepare for the Australian Open.

"He didn't have the same preparations this year and that could be one of the reasons for the injury.

"Also, the way he moves is so physical. It's not like Roger Federer, someone who glides around the court."

Nadal had an injury timeout during the fourth set but couldn't recover and pulled out at 2-0 down after having his serve broken in the decider.

Fellow pundit Miles Maclagan, who used to coach Andy Murray, added: "It wasn't an injury we saw coming.

"We've all seen problems with the knees but that seemed higher up in the hip flexor.

"From a selfish point of view we were cheated to the climax of the match there.

"Rafa puts his body through a lot and at this stage of the year you do see it.

"Players put in a lot of training but players are not battle-hardened and they aren't used to playing with tension and the nerves.

"Rafa is in a fair bit of discomfort there as they tried to release the muscles and the tension, whatever is causing the problem."