Immovable Nadal digs deep to seal No.1 ranking LONDON (Reuters) - The year dawned with a cloud hanging over Rafael Nadal's career but will end with him on top of the world after a win over Stanislas Wawrinka at the ATP World Tour Finals on Wednesday that illustrated why he is in the pantheon of tennis greats. He has certainly played better during a sensational season that began in February after a seven-month injury layoff, but the way he soaked up everything the Swiss could muster to complete a 7-6 (5) 7-6 (6) victory on Wednesday underlined Nadal's almost fanatical refusal to be beaten. Wawrinka walked off the cavernous showcourt wondering how he is yet to take a set off Nadal in 12 meetings, but it would be a harsh judge who criticized the world No.8 for failing to convert the fleeting chance that came and went in the second-set tiebreak. He was within a whisker of taking a high-caliber opener too, but Nadal, as he has in winning 74 matches in a spectacular season of almost total dominance on all surfaces, proved armor-plated in defense and ruthless when invited to attack. Nadal arrived in London for the glitzy season-ender needing two wins to fend off Novak Djokovic in his bid to end a year as world No.1 for the third time in his career. After dismantling compatriot David Ferrer on Tuesday, he was stretched to the limit by Wawrinka in his second Group A match, digging deep to notch up another milestone. By winning in straight sets he also assured himself of a place in the semi-finals here, and few would bet against the 13-times grand slam champion going on to claim the only significant title still missing from his CV. Nadal also finished 2008 and 2010 as world No.1 but this time his achievement seems more impressive considering where he was in February. After sitting out the second half of 2012 because of the knee injury that has dogged his career, Nadal missed the start of this season with a virus, and even when he re-surfaced on south American claycourts he was still wincing through the pain on occasions. The outlook looked gloomy, but nine months on he has arrived in London with 10 titles from 16 tournaments, reaching the final of three more, and while he has gone to great lengths to insist rankings are no longer his goal, the way he celebrated when Wawrinka struck a forehand into the net after two hours, 12 minutes spoke volumes. "Today was more important to secure the number one (ranking)," Nadal, whose only blemish so far in London was a warning for slow play, told reporters. I think after all that happened last year, I felt I deserved to be there at the end of the season. And today I did," Nadal added after proudly reeling off his 2013 achievements, which included five Masters 1000 Series titles, an eighth French Open crown, and the U.S. Open. This is one of the best things that I did in my career, to come back to number one after three seasons. That's very difficult in our sport, and after a very important injury. Now I can really be focused only on the tournament because the year end is over. Much has been made of Nadal's shift in emphasis to a more attacking style this year - a tweak designed to lessen the load on his knee and that brought him unprecedented success on the U.S. hardcourts that used to furrow his brow. Against Wawrinka, however, it was his trademark defensive skills that proved decisive. Wawrinka hit twice as many winners as Nadal and won more points, but when it mattered most, Nadal proved why he is one of the hardest men to put away. When the Swiss broke Nadal's serve with an exquisite forehand winner to level the first set at 5-5 and then nosed 6-5 ahead, the momentum had shifted his way. At 5-5 in the tiebreak Wawrinka had his foe pinned against the ropes, but somehow the Spaniard extricated himself with a desperate lob that forced a rash error from his opponent. When Wawrinka stumbled on the next point, Nadal darted in to swat away a forehand. Losing a 25th consecutive set to Nadal left Wawrinka staring at the equivalent of a sheer Swiss rock face, but he launched a stirring fightback from 4-1 down in the second only to see Nadal save a set point with a powerful smash. All is not lost for Wawrinka, who can still qualify for the semi-finals if he beats David Ferrer on Friday, providing Tomas Berdych does not beat Nadal in the final group games. "It's all about me for the next one," Wawrinka, who beat Berdych on Monday, told reporters. I showed today that I'm playing my best tennis, that I can really, really fight. Berdych kept his hopes of a semi-final spot alive with a solid 6-4 6-4 defeat of Ferrer, who looked weary after seven consecutive weeks on the road. Editing by Ed Osmond and Stephen Wood