While Andy Murray was confirmed as world number one on Monday, one of his predecessors, Roger Federer, slipped outside the top 10 for the first time since 2002.
An injury-blighted year has ended the 17-time grand slam winner's 14-year stay among the 10 highest-ranked players.
Federer started the year in third but underwent surgery on a torn meniscus in his left knee in February following a run to the Australian Open semi-finals.
The 34-year-old pulled out of the French Open with a back problem but returned to reach the semi-finals of Wimbledon, where he lost a thrilling five-set encounter to Milos Raonic.
However, Federer announced in July he would sit out the Olympic Games in Rio and the US Open in a bid to return in 2017 in the best possible condition.
The effect of his absence on the ATP Tour has seen him slip to 16th in the rankings, his lowest position since May 2001.
Federer is scheduled to return at the Hopman Cup in Melbourne in January.