Nadal is due to play Belgium's David Goffin in his first match at the tournament on Monday, but told reporters that his injured knee might not hold up.
"I'm doing my best," said the Spaniard, twice a runner-up but never champion of the season-ending event.
"I hope... (but) if I am not ready by Monday I won't be here. If something negative happens, that's it and you go home.
"(But) my plan is to play here and be competing well."
Nadal was forced to pull out of the Paris Masters last month with the knee injury but London tournament organisers, as well as fans with tickets, will be praying the world's top tennis player will be healthy enough to feature.
World number two Roger Federer said it would be a blow for Nadal to miss out.
"He's a superstar of our game. If he's here, that's a very good sign. It will be interesting but we hope he plays well," said the Swiss.
Nadal practised on centre court at the O2 Arena on Friday, hitting with Croatian Marin Cilic, who is also among the top eight ranked players competing. He shared jokes with coaches Carlos Moya and Toni Nadal on the court, but appeared frustrated at other moments with an injury that is clearly aggravating him.
(Editing by Christian Radnedge)
