The 25-year-old showed his softer side during a verbal rally with the Swiss world number one, as they geared up for the tournament at London's O2 Arena.
The pair swapped tennis whites for smart suits to attend the gala event at the Royal Courts of Justice along with the rest of the world's top eight players.
The Dunblane-born world number three, who took gold in the men's singles at the London Olympics and went on to claim his first Grand Slam title by beating Novak Djokovic at the US Open in September, is guaranteed a rapturous reception in London.
He has been drawn to face Djokovic in Group A, which also features Frenchman Jo-Wilfried Tsonga.
Federer heads Group B, which also contains David Ferrer, Juan Martin del Potro and Janko Tipsarevic.