Fighting Federer claws past del Potro into semis LONDON (Reuters) - Roger Federer's sublime artistry will define his career but in beating Juan Martin del Potro to reach the last four at the ATP World Tour Finals he showed that when the chips are down he can scrap like an alley cat. For most of Saturday's Group B shootout for the right to play Rafa Nadal, the 17-times grand slam champion looked to be heading for the exit but the Swiss refused to buckle under the Argentine's brutal power and clawed out a 4-6 7-6 (2) 7-5 win. It has not been a vintage season for Federer, who has dropped to world number seven, but he could yet end it on a high, although he must get past world number one Nadal who won Group A with a 100 percent record. Federer's enthralling victory over del Potro sealed runners-up spot in Group B behind Novak Djokovic who will face Federer's fellow Swiss Stanislas Wawrinka in the other semi. Before that Djokovic faces Richard Gasquet in a dead rubber later on Saturday. It was a display of wild fluctuations from Federer against an opponent to whom he had lost three times in a row before a confidence-boosting victory at last week's Paris Masters. He admitted he had been a little fortunate to survive. "A lot of coming from behind, trying to get back into the match, and it was amazing that I was able to pull it off," a delighted Federer said on court. I could never play free-flowing tennis. I fought hard, maybe got a bit lucky at times. The 32-year-old Swiss seemed to sacrifice substance for style as he casually lost serve twice to trail 5-1 in the opening set before he almost repaired the damage with an exhilarating comeback that fell just short when del Potro aced his way out of trouble when serving at 5-4. Six-times former champion Federer had only once fallen in the round-robin phase of the season-ending event in 11 previous appearances and never since it switched to London in 2009. He found himself a break down in the second set too but reeled off 12 consecutive points as the pendulum swung his way and then played a majestic tiebreak to level. Federer dipped again at the start of the deciding set when a netted forehand gave del Potro 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 break back in the fifth game. The sell-out crowd roared their approval at 4-4 when del Potro produced an incredible lob played between his legs and Federer replied with a smash over his shoulder before finishing the point with a whipped forehand winner. Del Potro responded to hold but with Swiss cowbells echoing around the arena, Federer held his nerve to make it 5-5 and then pounced to gain the decisive break for 6-5. Even then del Potro looked like forcing a deciding set tiebreak but Federer would not be denied and belted down his 10th ace to complete a remarkable victory. "I don't get a day off, I've got to back it up and be ready to go again tomorrow," Federer said. I'll try to make his (Nadal's) life a bit difficult. Reporting by Martyn Herman, editing by Tony Goodson