By Martyn Herman
LONDON, Nov 9 (Reuters) - When Roger Federer hangs up his rackets his sublime artistry will be his lasting legacy but Saturday's win over Juan Martin del Potro at the ATP World Tour Finals showed when the chips are down he can also fight like an alley cat.
For most of Saturday's Group B shootout for the right to play Rafa Nadal in the semi-finals, the 17-times grand slam champion was heading for the exit but the Swiss refused to budge under the Argentine's brutal power and clawed out a thrilling 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 is ending it on something of a high and insists he will come out swinging when he faces world number one Nadal for a place in the final.
"I feel very happy and very upbeat," Federer, who was behind all the way against del Potro until 6-5 in the deciding set, told reporters. "I'll give it one more big shot here tomorrow and hopefully I'll make it to the finals."
Federer's enthralling victory over del Potro sealed the runners-up spot in Group B behind Novak Djokovic who will face Federer's compatriot Stanislas Wawrinka in what could be a landmark day for Switzerland.
Wawrinka would have enjoyed the sight of Djokovic grafting for more than two hours late on Saturday to beat Richard Gasquet 7-6(6) 4-6 6-3 in a dead rubber, albeit one that earned the Serb another $142,000, 200 ranking points and extended his winning streak to 20 matches.
Federer, who has won only two of his last nine matches against Nadal, will have to go up a notch after a display of wild fluctuations against del Potro, an opponent to whom he had lost three times in a row before a confidence-boosting victory at last week's Paris Masters.
An error-strewn first set saw him go 5-1 down and he always seemed to be fighting himself as well as his opponent before his class finally shone through.
"There was 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.
"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 in the opening set before an exhilarating comeback fell just short of repairing the damage.
He seemed poised to get back to 5-5 but del Potro twice found huge first serves to save break points before pocketing the opening set.
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 its spectacular London setting 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. He then played a majestic tiebreak to level.
Inexplicably, Federer dipped again at the start of the deciding set when a netted forehand gave del Potro a 2-0 lead.
"I was really getting angry at myself, thinking how I could just play a loose game, why is this happening? I'm trying to focus so hard for it not to happen, and it did," Federer said.
However, he channelled his anger into some forceful tennis to put del Potro on the defensive and broke back when the Argentine's erratic forehand malfunctioned in the fifth game.
Some of the tennis was scrappy but the sell-out crowd roared its approval at 4-4 when del Potro produced an incredible lob played between his legs and Federer replied with a smash on the run over his shoulder before finishing the point with a whipped forehand winner.
It set the stage for a thrilling finale to the match which had the atmosphere of a Davis Cup tie with Swiss cowbells jingling around the stands and chants of "Roger" ringing out.
A cool Federer served to stay in the match at 4-5 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."
Del Potro was left to rue a match that got away but was encouraged by his return to the top five after some injury-plagued years since winning the 2009 U.S. Open.
"I think I'm doing great matches against the top guys," said 25-year-old del Potro, who has posted wins against Nadal, Djokovic, Andy Murray and Federer this year.
"I think I am the only player who beat all of them in this season, which means something for me." (Reporting by Martyn Herman, editing by Alison Wildey)