Rafael Nadal
Nadal has still not completely recovered from a knee problem Getty

Doubts have emerged over world No.1 Rafael Nadal's fitness ahead of the 2018 season after he pulled out of the Abu Dhabi exhibition event due to continued discomfort in his knee.

Nadal ended his spectacular 2017 season when he withdrew from the ATP Finals in London after his knee issues flared up again during his round-robin 7-6, 6-7, 6-4 loss to David Goffin back in November.

The Spaniard was expected to return to action at the Mubadala World Tennis Championship, which takes place in Abu Dhabi from 28-30 December before the start of the 2018 season.

However, he has withdrawn from the event and has been replaced by fellow compatriot Roberto Bautista Agut, who broke the news on Twitter.

"Great news for me to share with you that I will compete in the exhibition tournament in Abu Dhabi," Agut tweeted.

"A great way to start the season. I hope that Rafa gets well soon!"

According to Spanish publication Marca, the 31-year-old is still experiencing discomfort in his knee and reportedly cancelled his training week scheduled for 18-22 December at his academy in Mallorca.

It only adds to the concerns that Nadal may not be fully fit for the new year after there were already doubts on his participation at the 2018 Australian Open.

"In Paris, the injury was already pretty bad," Greg Rusedski said in November. "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 [ATP Finals], then you see him grimacing like that. You wonder how much this will set him back for 2018."