By Martyn Herman
LONDON, Nov 6 (Reuters) - Rafael Nadal fought off a ferocious challenge from Stanislas Wawrinka to earn his second Group A victory at the ATP World Tour Finals on Wednesday and guarantee ending 2013 as world number one.
The Spaniard arrived in London needing a minimum of two victories at the year-ending showpiece and, after breezing past David Ferrer on Tuesday, he was stretched to the limit by Swiss Wawrinka in his second round-robin match before coming through 7-6(5) 7-6(6).
His straight-sets victory means 13-times grand-slam champion Nadal is assured of a semi-final spot at the event which he has never won, while tournament debutant Wawrinka can also still make the last four if he beats Ferrer on Friday.
Nadal served for the first set at 5-4 but Wawrinka broke with some dazzling attacking play and then went toe to toe with the top seed in the tiebreak before an unfortunate stumble as he approached the net at 5-6 down allowed Nadal to crunch a forehand winner away and pocket the opener.
Wawrinka delighted the crowd as he battled back from 4-1 down in the second set and seemed poised to take it to a decider when he held a set point in another tense tiebreak only to succumb to Nadal for the 12th time in their 12 meetings, during which he has never won a set. (Editing by Clare Fallon)