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Tennis - Shanghai Masters tennis tournament - Semi-finals - Shanghai, China - October 14, 2017 - Rafael Nadal of Spain in action against Marin Cilic of Croatia. Reuters /Aly Song

World No. 1 Rafael Nadal has yet to recover from his season-ending knee injury, raising serious concerns for the Spaniard ahead of the 2018 Australian Open and Brisbane International. Nadal has already pulled out of the Mubadala World Tennis Championship, a year-ending exhibition tournament in Abu Dhabi.

A year ago, Nadal made an incredible comeback to the sport at the Australian Open, after enduring a career-threatening wrist injury in 2016. Nadal pushed arch-rival Roger Federer to five sets in the finals at Melbourne Park before capturing his 10th French Open title and third US Open title to increase his Grand Slam tally to 16.

Despite a renaissance year, injuries halted Nadal's momentum at the end of 2017. After pulling out of his Paris Masters quarterfinal match against Serbia's Filip Krajinovic, Nadal played through a knee injury against David Goffin before withdrawing from the Nitto ATP Finals in London. At the time, Nadal was confident about recovering in time for the Brisbane International and potentially the Abu Dhabi Open.

2018 Australian Open: Is Rafael Nadal seriously injured?

Last week, Nadal cancelled a five-day training camp in Majorca. Nadal told Joao Sousa, his training partner, that he wasn't ready for practice. "He called me to tell me he was not feeling well," Sousa said, according to Sydney Morning Herald.

“It was a tough 2017 and I need to take my calendar in a different way in order to be ready. For this reason, I have sadly announced to the organizers and now to you fans that I won’t play this time in Abu Dhabi," Nadal told the official website of the Abu Dhabi exhibition event.

Meanwhile, Serena Williams has confirmed her participation in Abu Dhabi later this month. She will face off against reigning French Open winner Jelena Ostapenko Saturday. Williams hasn't played competitive tennis since the birth of her daughter, Alexis Olympia. Williams has also been medically cleared to defend her Australian Open crown later in January.