Novak Djokovic stormed through to the quarter-final of the Australian Open with a convincing win over Adrian Mannarino on Sunday.
However, former player Paul McNamee has raised a concern that the world No 1 is still struggling with illness heading into the quarter-final.
Djokovic handed out a double-bagel to Mannarino before eventually winning in straight sets on Rod Laver Arena.
It was the Serbian's most convincing performance of the tournament so far as he goes in search of a record-extending 11th title.
Novak Djokovic fitness concerns have been raised ahead of the Australian Open quarter-final
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Djokovic revealed he had an illness coming into the tournament, which hampered him in the opening rounds.
He insists he is now in a better condition health-wise than he was a week ago.
But former world No 1 double star McNamee still has his concerns.
During Djokovic's win over Mannarino, McNamee posted his worries on his X account.
Early on in the match, he wrote: "Novak is up a break, but gee he’s really struggling physically… not looking well at all… mm."
McNamee still held the same view later in the match despite Djokovic seemingly easing to victory.
He claimed the 36-year-old's backroom staff were visibly concerned as they watched on from the stands.
McNamee praised Djokovic's mental strength to fight on but reckons it could catch up with him later in the tournament.
He wrote: "In all my time, I’ve never seen a player box so concerned at 5 love up first set than Goran [Ivanisevic] & co in Djokovic’s box…
"He’s clearly not well…what mental fortitude to be able to push through it, but there will be limits."
Djokovic is due to play Taylor Fritz in the quarter-final on Tuesday.
It wasn't just McNamee who noticed the 24-time Grand Slam champion was not at his physical best.
Jim Courier said while commentating on Channel 9: "He is without question physically wounded at the moment.
"He is sick. He told us he was sick five days before the tournament. You would have thought by now he would have healed up.
"Something is still not right inside his body because his energy levels are not what we are used to seeing.
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In all my time, I’ve never seen a player box so concerned at 5 love up first set than Goran & co in Djokovic’ box… he’s clearly not well…what mental fortitude to be able to push through it, but there will be limits
— 🎗️ Paul McNamee (@PaulFMcNamee) January 21, 2024
"It may not show it on the scoreboard … (but) Novak is showing his mental toughness here. He's trying to be as efficient as possible."
Djokovic played down concerns over his health after his fourth-round win and was delighted with how he played.
He said: "The best sets I’ve played in a while.
"I really wanted to lose that game in the third set because the tension was building up so much in the stadium.
"I needed to get that out of the way and refocus on what I needed to do to finish the match.
Novak Djokovic has made it through to another Australian Open quarter-final
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"I served very well. In the moments when I needed to come up with the first serve, I did. All in all, great performance.
"The last couple of days has been really good so it's going in a positive direction health-wise, tennis-wise, so I'm really happy."