Kei Nishikori admits he's struggling to recover from two weeks in hard quarantine in his against-the-odds bid to make an impact at the Australian Open.
The former US Open finalist was the biggest name of the 39 men's stars holed up in their hotel rooms for a fortnight as the rest of their Open rivals were allowed up to five hours a day to train.
A four-time quarter-finalist at Melbourne Park, Nishikori has dragged himself out of the brutal lockdown to lead Japan at the second annual ATP Cup getting underway on Tuesday.
"I don't know how it's going to be. I just try to play my best tennis," the 31-year-old said on Monday, trying to look on the bright side of his time shut away.
"At least I have two matches, singles, maybe doubles, I don't know.
"But I feel OK after being two weeks in a room. It is obviously not easy.
"My body is reacting very tough. Very sore everything. But I have still two more days, so I should be OK."
While Nishikori was looking for positives, Japan captain Max Mirnyi said it was impossible to know how the one-time world No.4 would fare against Russia's 2020 ATP Finals champion Daniil Medvedev on Wednesday.
"Being in the room and not being able to be outside doing exercise and breathing fresh air, we don't know how the body will react," Mirnyi said.
"Obviously it's a big challenge. We did, I think, the maximum we could do being in a room environment, which Tennis Australia provided pretty much anything the players have asked for.
"There was a lot of physical activity going on. My room was next to Kei's room. I could see there was a lot of medicine-ball activity, weights, running and jumping.
"Again, there is only so much you can do in a room."
Mirnyi said it was equally as challenging for Nishikori since leaving his hotel room.
"First couple practices outside have been quite difficult with just having to breathe in all the air and move, looking at the bright sun of Melbourne," he said.
"But, listen, we do what we can. There's a few more other players in a similar situation.
"We're all going to be just happy and glad that the tournaments are here for us to play.
"Hopefully by the time the tournament starts, everybody's going to be the best possible ready."
While Nishikori and Japan don't start until Wednesday, Australia and Novak Djokovic's defending champion Serbs, up against Canada, are in action on day one of the Cup.
Djokovic, also the Australian Open titleholder, faces Denis Shapovalov in his season opener while Alex de Minaur and John Millman will lead Australia into battle against 2020 runners-up Spain.
De Minaur plays Rafael Nadal in a Tuesday night blockbuster and repeat of his three-set ATP Cup semi-final loss to the 20-times grand slam champion last year.
Millman meets 2019 Wimbledon semi-finalist Roberta Bautista-Agut, with Australia very much the underdogs.
In Tuesday's other ties, Austria, led by reigning US Open champion Dominic Thiem, play Italy and Russia take on Argentina.