Rafael Nadal
Rafael Nadal waves to the crowd after his loss to David Goffin at the ATP Finals Getty

World number one Rafael Nadal is in danger of missing out on the 2018 Australian Open after pulling out of the ATP Finals for the sixth time.

The Spaniard, who had withdrawn from the Paris Masters due to a knee injury, took part in London as he hoped to end the year on a high and win one of the few events he has not tasted success in yet.

However, Nadal's knee issues flared up again during his round-robin 7-6, 6-7, 6-4 loss to David Goffin on 13 November, resulting in his eventual withdrawal from the competition.

"My season is finished," Nadal said after the loss. "Yeah, I had the commitment with the event, with the city, with myself. I tried hard.

"I did the thing that I had to do to try to be ready to play. But I am really not ready to play."

The 31-year-old may now have set himself back for 2018, according to former British number one Greg Rusedski, with the Australian Open taking place in January.

"In Paris, the injury was already pretty bad," Rusedski told Sky Sports via The Mirror. "Maybe he took painkillers or got an injection and said, 'I'm going to play one match and that's it.' In retrospect, you wonder whether that was the right decision.

"He's accomplished everything in the sport apart from winning this event, then you see him grimacing like that. You wonder how much this will set him back for 2018."

Former American doubles star Peter Fleming understands why Nadal may have risked aggravating his injury in London.

However, he also believes the 16-time Grand Slam champion probably thought long and hard about his participation, even if it means he will be disappointed about potentially missing out on playing in Melbourne.

"Yeah (you can understand why he wanted to play) and don't you think that's his prerogative?" Fleming explained. "He can do whatever he wants to do.

"If it means that he won't be able to play in Australia, obviously he'd be disappointed, everyone will be disappointed.

"But he doesn't lose a lot of sleep, I don't think, about decisions that he's made. He thinks long and hard about something and once he's committed to something, it's done and it's over. So I don't think we should worry about him, he'd be fine."

If Nadal does miss out on the Australian Open, his world number one ranking is also at risk.

With Roger Federer potentially lagging just 140 ranking points behind him at the start of the new year if the Swiss ace remains unbeaten in London, Nadal could fall further behind him if he fails to defend his 1,200 points at Melbourne.