Rafael Nadal on Tuesday announced he will miss this month's Swiss Indoors tournament, boosting arch rival Roger Federer's dream of ending the season as world number one.
The 16-time Grand Slam winner told his Facebook followers he was skipping the October 21-29 tournament due to a knee injury.
"I sadly announce that I have to pull out of the Swiss Indoors Basel, after seeing my doctor in Spain," he wrote.
Federer beat the Spaniard at the Shanghai Masters on Sunday to close the gap at the top of the ATP rankings.
Federer has closed to within 1,960 points of Nadal with 500 on offer in Basel followed by the indoor tournament at Bercy in Paris and the end-of-season ATP Finals in London.
In his social media statement, Nadal said he had seen his doctor on his return from China.
"I am suffering from an over-stressing of the knee and the problems were already present at the tournament in Shanghai which now forces me to take a time off on advice of my doctor.
"After two great weeks in China, with the title in Beijing and the final in Shanghai it is time for some rest.
"I want to send a special message to the many tennis fans in Switzerland which have always showed great support and respect also on my matches with Roger. Hope to see you next year."
Federer put on a masterclass to beat Nadal 6-4, 6-3 on Sunday, the 36-year-old lifting his sixth trophy this season and underlining a turnaround in fortunes against his long-time nemesis.
The world number two ended a sequence of five straight defeats to Nadal at Basel last year, before winning their gripping Australian Open final in January.