Shanghai - Juan Martin del Potro said that he is suffering from a virus and reviewing his health "hour by hour", after labouring into the Shanghai Masters last 16 on Wednesday.
The Argentine world number four lost Sunday's final of the China Open in Beijing to unseeded Nikoloz Basilashvili and afterwards revealed that he had been unwell.
Three days on and the 30-year-old continues to be under the weather, but just about did enough to see off France's Richard Gasquet in Shanghai, 7-5, 7-6 (9/7).
He is not the only one feeling off colour - fifth-ranked Alexander Zverev has a sore throat and blocked nose.
Between coughs at his post-match press conference, Del Potro said: "It wasn't easy to play for me today, I did what I can. I didn't run too much.
"But of course I would love to feel better, first of all to myself, and then to play tennis, but it's going to be difficult because I'm not taking time to recover 100 percent."
Del Potro said that he had decided to withdraw from last Saturday's China Open semi-final against Fabio Fognini - only for the Italian to pull out before him with an ankle injury.
"I'm in Asia, of course I want to play tennis here, but I don't want to put at risk my health also.
"But I'm going like hour by hour and see how I'm feeling," added Del Potro, who plays 13th seed Borna Coric next in Shanghai.
"I got lucky that I won, but tomorrow will be a different day.
"If I have a good night today and I have a good recovery tomorrow morning, maybe I will feel a little bit better for tomorrow night and play better than today."