Only the top eight players qualified for the season-ending tournament, which begins Sunday at the O2 Arena in London.
The top two finishers from each round-robin group will advance to the semifinals.
Federer, a five-time winner of the season-ending event, enters the tournament on a 12-match winning streak. He beat Tsonga for the Paris Masters crown last Sunday and also won in his native Basel the previous week, adding a couple of Davis Cup wins to his run of 10 straight ATP victories.
Djokovic is the only other former champion in the field, having captured the crown in 2008. He will be trying for a 12th title in 2011, an incredible season that has included Grand Slam titles at the Australian Open, Wimbledon and the U.S. Open.