By Martyn Herman

LONDON (Reuters) - Spain's Rafael Nadal is guaranteed finishing the year on top of the rankings after winning the French Open, Wimbledon and the U.S. Open, the ATP said on Thursday.

The 24-year-old has been virtually unstoppable since the beginning of the European claycourt season and on Monday completed his career grand slam by beating Novak Djokovic to win the U.S. Open at Flushing Meadows. He is the youngest player to have won all four grand slams in the professional era.

"It has been an incredible season -- one of my best ever, if not the best," Nadal said in a statement on Thursday.

"Winning the U.S. Open together with Roland Garros and Wimbledon, as well as the three back-to-back (ATP World Tour) Masters 1000s in Europe, was not easy.

"I worked very hard to get back to the top and it feels really good to know I will end the year as number one."

It is the second time in three years that Nadal will finish the year as number one and were it not for an injury-plagued 2009 when he could not defend his Wimbledon crown he could have topped the rankings three years in a row.

Nadal and Roger Federer are the only players since 2000 to clinch the year-end top spot so early, with Swiss Federer achieving the feat in 2004 and 2006.

The Mallorcan will be officially crowned as the 2010 ATP World Tour Champion at the season-ending World Tour Finals in London in November.

(Reporting by Martyn Herman)