Djokovic underwent knee surgery after he was forced to withdraw from the French Open, and at January’s Australian Open, he injured his hamstring.

The 37-year-old retired after the first set of his Australian Open semi-final as a result, Stubbs is starting to wonder if this will start becoming a recurring problem for Djokovic.

Rennae Stubbs shares her concerns about Novak Djokovic’s fitness moving forward

Novak Djokovic has done a phenomenal job in keeping himself fit to the point where he is still able to compete at a very high level into his late thirties.

But as he gets older the chance of suffering from injury only increases as the 24-time Grand Slam champion has experienced over the past year.

Therefore, following Djokovic’s loss to Matteo Berrettini at the Qatar Open, Rennae Stubbs is beginning to question whether he still has enough physical capacity and whether his body can recover from these setbacks.

“You have to start asking the question – Novak, physically, I know he has had his issues in Australia with the leg, he didn’t look great leaving the Middle East,” she said on the Rennae Stubbs podcast.

“He has been struggling with injuries over the last couple of years now, with the knee and the leg and I’m telling you… once you get to a certain age you don’t bounce back. And, yes, Novak has bounced back better than most, better than probably anyone.

“But this is starting to be a bit of an issue now. Not winning matches and then when you haven’t played for a while, then you come back and you’re older.

“I don’t care how much you train and how good you look on the practice court, when you go and play a match, you are pushing yourself more and doing more. You are more nervous, your body is more uptight and he is struggling more and more and more.”

What was Novak Djokovic’s fitness update after the Qatar Open?

The Qatar Open was Djokovic’s first match since retiring from the Australian Open semi-finals due to injury.

It appeared as though the hamstring injury would keep out of action for some time, but he made a miraculous recovery to return in time for the ATP 500 event.

He succumbed to a 6-7, 2-6 defeat to Berrettini, and post-match Djokovic said he played without pain.

“I didn’t have any pain or discomfort in that sense. I was outplayed by just a better player today,” the former World number one told reporters.

“Yes, I wasn’t at my desired level, and it could be that I’m still not moving the way I want to move, but, I mean, I played without pain, so there is no excuse in that.”