London, England -- (Sports Network) - Roger Federer battled back from a set down to defeat Spaniard Fernando Verdasco in the evening session of the season- ending ATP World Finals.

The French Open and Wimbledon champ dropped the first set before rallying for a 4-6, 7-5, 6-1 victory.

Scotland's Andy Murray notched a 6-3, 3-6, 6-2 victory over U.S. Open champ Juan Martin del Potro in the first round-robin match of Group A on Sunday.

The first five games of the opening match went to Murray, but he stumbled in the second set and del Potro was able to open a 3-0 lead on the way to forcing a decisive third. Murray, though, broke in the second game of the final set en route to a 4-1 lead and cruised to the finish.

Group B play starts Monday in the exclusive eight-man field. Australian Open champ Rafael Nadal will meet French Open runner-up Robin Soderling in one of the day's matches, while Novak Djokovic and Nikolay Davydenko will square off in the other.

Federer will try to finish as the year-end No. 1 for the fifth time in six years, while Nadal has a shot to capture the honor for a second straight season.