Federer dropped from ninth to 16th when the weekly rankings were released, marking the Swiss' lowest position since he sat 13th in October 2002.
Although Federer won the last of his record 17 majors a while ago in 2012, his slide is more the result of injury than a slump brought on by advancing years.
He reinjured his knee at Wimbledon in July in a five-set semifinal loss to Canada's Milos Raonic -- when he was ranked third -- and a few weeks later decided to end his season to focus on 2017.
The decision sparked speculation about whether Federer, who turned 35 in August, might simply quit the sport but he was adamant that he still wanted to play for "another few years."
He told Swiss newspaper Tages Anzeiger last month that he was on course to return in January, with his first event scheduled to be the Hopman Cup, a team event, in Perth.
