Roger Federer delays comeback from injury as he pulls out of IPTL

Federer has dropped to 16th in the world rankings

Roger Federer will not return from a knee injury until the new year, having pulled out of the International Premier Tennis League (IPTL) because of "uncertainties" surrounding the event.

The 35-year-old Swiss has not played since a Wimbledon semi-final defeat by Milos Raonic in late July.

World number two Serena Williams has also withdrawn from the IPTL.

This year's tournament features one fewer team than last year, and has been undermined by financial difficulties.

Federer, a 17-time Grand Slam champion, is expected to be fit for January's Australian Open.

"We have had challenges this year, and we were hoping to get past them," said IPTL founder Mahesh Bhupathi.

"With the current economic climate in India and the uncertainty of spending money, I reached out to both Roger and Serena to explain the situation."

What is the IPTL?

Now in its third year, the IPTL is a team competition featuring four city franchises - Indian Aces, Japan Warriors, Singapore Slammers and UAE Royals.

Teams face each other in fixtures in Tokyo, Singapore and Hyderabad from 2-11 December.

There are seven players in each team, with each fixture played over five sets comprised of a men's singles, women's singles, men's doubles and mixed doubles match before a 'past champions' singles, featuring iconic players.

Kei Nishikori, Tomas Berdych and Nick Kyrgios are the only top-20 singles players involved this year, while 'past champions' include Mark Philippoussis and Goran Ivanisevic.

World number one Andy Murray played in the 2014 and 2015 tournaments.