By Martyn Herman
LONDON (Reuters) - Roger Federer set up a mouthwatering semi-final with Rafael Nadal at the ATP World Tour finals after battling past Juan Martin del Potro in their Group B shoot-out on Saturday.
The Swiss seemed to spend most of the match fighting from behind but showed the grit of a true champion to topple the Argentine 4-6 7-6(2) 7-5 at London's o2 Arena.
Victory meant he qualified for the last four as group runner-up to defending champion Novak Djokovic who will face Federer's fellow Swiss Stanislas Wawrinka in Sunday's other semi-final.
Federer had only once fallen in the round-robin phase of the season-ending tournament in 11 previous appearances and never since the tournament switched to London in 2009.
However, Del Potro looked favourite to tarnish that proud record as he moved a set and a break ahead.
Federer produced a stirring fightback from 5-1 down in the opening set only to fall short but hit back from a break in the second to level the contest with a commanding tiebreak.
The level of Federer's game fluctuated throughout the match and it dipped again at the start of the deciding set when a netted forehand gave the powerful Argentine a 2-0 lead.
Again Federer responded to a deficit with some of his most forceful tennis and Del Potro's erratic forehand gifted the six-times champion two cheap points in the fifth game to get the set back on serve.
With the crowd roaring on the six-times champion he broke Del Potro's serve to lead 6-5 and then completed his second victory over Del Potro in consecutive weeks with an ace.
(Reporting by Martyn Herman, editing by Alan Baldwin)