Zverev still struggling with lungs after pneumonia

World No.2 Alexander Zverev says 25 per cent of his lungs weren't working after having pneumonia in late September.

"I ended up in hospital before the Laver Cup when I had a fever of 40 degrees celsius for three days in a row and at some point I felt like I couldn't breathe," he said after his win in the ATP Finals opener on Monday.

Zverev has been suffering from the effects of pneumonia for some time, complaining of fever and a cough at the Laver Cup in Berlin in September.

He subsequently withdrew from the China Open in Beijing and failed to reach the round of 16 at the Shanghai Masters, but won the Paris Masters earlier this month.

"I'm getting tired a bit more quicker. But I'm feeling okay. Which doesn't mean that my lungs are 100 per cent but right now I feel fit again," he said after the 6-4 6-4 win against Andrey Rublev.

Zverev had previously said he expects to be 100 per cent fit for the Australian Open in January.