Novak Djokovic is concerned about Andy Murray slipping on the grass on his return at the Fever-Tree Championships but said the mental challenge will be his biggest to overcome.
Murray has not played since Wimbledon last year, but the three-time grand slam champion will make his long-awaited comeback against Nick Kyrgios in the first round at Queen's Club next week.
The Briton underwent hip surgery in January and Djokovic - who will also start his grass-court season at the same event - said it is not just a physical test the former world number one will face when he returns.
Djokovic, who has also been hampered by injuries, said: "He needs that support [from the Queen's crowd]. He needs people to back him up, to be behind him and show him that he was missed.
"I know how that feels. Having a couple of weeks at home, these are the events where he will give 100 per cent to try to get out on the court.
"He has been absent for longer than I have. I don't know how painful it is still, or not, for him to move around.
"But I think the biggest challenge will always be mental. In my situation that was the case. How to get it out of your head, understand that it's behind you, that you're fine now, that you're healthy and you can focus on your game rather than thinking 50 per cent of the time about whether or not something can happen.
"Does it hurt me? Does it not hurt me? Am I imagining things? Is it real? Is it not real? Then 50 per cent of the time you are thinking about your tactics and what you have to execute.
"If you don't have mental clarity on the court, especially on grass where everything happens very quickly, it's very difficult to play."
The Serbian added: "I don't know the state of his hips. But the one thing that could be a bit dangerous is slipping on the grass.
"That's something that is very unpredictable and grass is always this kind of surface where one wrong footing can make something go wrong, especially in the hips.
"I just hope for his sake that doesn't happen."
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