LONDON.  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.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.ONLY BLEMISH"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.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.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.