Rafael Nadal secured the year-end world number one spot - and booked his place in the last four of the ATP World Tour Finals - with a straight sets victory over Stanislas Wawrinka at the O2 Arena in London.

The Spaniard was forced to work hard for his victory against the in-form Swiss star however, with a 7-6, 7-6 win taking over two hours in-front of a packed crowd in London.

It's a result that means Novak Djokovic can't overtake his rival at the top of the rankings, although the pair remain on a collision course for the final on Monday night.

Nadal has been in imperious form since returning from injury in February, claiming two Grand Slam titles, and is now in prime position to lift another title before the end of the year.

The 27-year-old has claimed consecutive straight sets victories in less than 24 hours, emphatically beating David Ferrer (6-3, 6-2) before a much tougher battler earlier today (Wednesday).

Nadal won a tight tie-break in the opening set, as Wawrinka boiled over following yet more disappointment against a player he's now lost to on 12 consecutive occasions.

A warning for racquet abuse didn't help matters, before a complaint was made against noise coming from Nadal's camp at the side of the camp.

No action was taken, Nadal secured the break and everything appeared to be going to plan. However, Wawrinka called upon his recent fine form to break back, and send the game to a second tie-break.

And, he had his chance when, at 6-5 up, his defensive return was seized upon by Nadal. The Spaniard didn't give his opponent another sniff, and is now in the final four ahead of his final Group A match against Tomas Berdych.

With the world number one spot already in the bag, the only thing that appears likely to stop Nadal from lifting the trophy this week is complacency.

Do YOU want to write for GiveMeSport? Get started today by signing-up and submitting an article HERE: http://gms.to/writeforgms