Roger Federer has fresh hope of finishing 2017 as number one after Rafael Nadal pulled out of the Swiss Indoors in Basel.
In October, the 31-year-old Spaniard - currently ranked number one in the world - won the China Open and reached the final of the Shanghai Masters.
Federer, 36, beat Nadal in the final of the Shanghai Masters but back-to-back deep runs at hard court tournaments caused the second most decorated man in the sport's history to strap-up the patellar tendon on his right knee.
The head-to-head record between the two legends still stands comfortably in favour of Nadal, who has won 23 of their 38 meetings.
Nadal shared details regarding his "over stressing of the knee" with his followers on social media.
"I sadly announce that I have to pull out of the Swiss Indoors Basel, after seeing my doctor in Spain just after landing from Shanghai," wrote Nadal on Facebook.
"I am suffering from an over stressing of the knee and the problems where 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 is mathematically able to catch Nadal and could become the number one ranked player for the first time since 2012.
"I mean, I don’t even know what it takes to be honest [to become world No. 1]," he told Sky Sports after beating his long-time rival. "I just know the World Tour Finals is a must and probably Paris too and probably Basel so that all depends on how Rafa plays.
"It’s a wild thought, that’s why I’m not even thinking about it to be quite honest at this point."