Rafael Nadal, the world number one, has dropped the strongest hint yet that he could pull out of the ATP World Tour Finals after admitting he is not yet certain of participating in the season-ending tournament in London.
The Spaniard withdrew from the Paris Masters last week with the recurrence of his long-term knee problems.
Angel Ruiz Cotorro, Nadal's personal doctor, has advised the 16-time grand slam winner to reconsider competing at the O2 Arena. And the 31-year-old has now suggested his participation is in doubt.
"I don't have to make a decision [yet]," said Nadal, speaking on Friday [10 November], 72 hours before his opening match. "I'm going to play – that's what my feeling is today. I can't predict what might happen in the next couple of days, but my feeling is that I'm here to play and to try my best."
He added to BBC Sport: "If I didn't believe I could be ready for Monday I wouldn't be here. The knee is not perfect, of course, after having to retire from Paris.
"In a couple of days you cannot expect to be 100% but I had treatment and I'm expecting to be better and better every day. I'm here to try my best to give me a chance."
Nadal begins his campaign against David Goffin on the second evening of competition on Monday [13 November]. Dominic Thiem and Grigor Dimitrov are his other round robin opponents as he bids to win the final showdown for the first time.
Tournament favourite and six-time winner Roger Federer has meanwhile enjoyed a far more tranquil build-up having missed the Paris Masters after winning the Swiss Indoors, to ensure he was fully fit for the London showcase.
The 19-time major champion played an exhibition match against Andy Murray in Glasgow in the week as a tune up for the ATP Finals and arrives in the English capital with a clean bill of health.
"I feel fine," said the 36-year-old. "I had a very slow last 10 days. I practised twice during the week in Paris and I only hit for an hour both times.
"I came here on Monday, then played the exhibition against Andy [Murray] in Glasgow and was then off again on Wednesday so I've been really taking it slowly to really just make sure I totally rested after Basel."