Novak Djokovic believes most tennis players are experiencing "mental issues and motivational issues" due to the restrictions imposed by the Covid-19 pandemic.
Most tournaments have been played without fans over the last year and players have been living in bubbles while competing, often unable to leave their hotel rooms freely.
Benoit Paire has expressed his unhappiness with the situation on several occasions while other players have said they have found it tough.
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With no immediate end to the restrictions in sight, world No 1 Djokovic has acknowledged that the majority of players are finding it hard.
"I think most of the players on the tour are experiencing some of the mental issues and motivational issues," he said.
"Playing in front of empty stands, travelling and not being able to go out of their hotel room. Just being restricted to the tennis courts and the hotel.

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"If you do it for a week it is fine, we can take that. But if you keep doing that week in, week out then it's a very challenging thing mentally and emotionally for anybody."
Dominic Thiem was due to play alongside Djokovic at the Serbia Open this week but has revealed that he has been finding life on tour difficult.
"I do understand, I do feel for him and for everyone," reflected Djokovic. "I have felt that myself at the end of the last season, playing in indoor events in front of empty crowds. It's just not something that we are used to.
"On the mental side of it, obviously I've heard that he is struggling with how he is feeling, he's a bit exhausted with the restrictions in the bubble life on the tour. Well, I understand what he's been going through because we all go through that."
Djokovic, who lost in the last 16 of the Monte-Carlo Masters last week, will face Sonwoo Kwon in the second round of the Serbia Open.
He says that although things are tough right now, hopefully there is a light at the end of the tunnel.
"It's a new experience. We have to accept it, we have to embrace it, and try to make the most out of it for us, and for our tennis. And then, hope and pray that crowds will be back very soon and we will be able to move freely. That would go a long way to help a lot of players."

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Djokovic also responded to Rafael Nadal’s comments that he was "more obsessed" with Grand Slam records than some of his rivals.
The world No 1 said: "Personally, I do not feel that I am obsessed with anything in life; what I feel is passion and huge desire. I am going towards achieving my goals and I have never had a problem verbalising it.
"Maybe someone cannot say something and then stick to it, but I never found it hard to say: ‘I want to break that record or reach a certain goal’. I do not know why that would be a bad thing, not just in terms of records, but anything, politics in tennis for instance.
"Ever since I was a young player, I did not fear voicing my goals: I wanted to be number one and to win Slams. Of course, those goals are growing – when you fulfil one, another one appears. It is my path, and it is unique, just like Rafa’s."
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