Nadal was atop the rankings at the end of 2008 and 2010. If he can hold off Novak Djokovic's slim chances of passing him in the rankings, Nadal would be the first player to regain the No. 1 spot and hold it at year's end.
Nadal is 71-6 in 2013 with 10 titles. He's made the finals in 13 of the 15 tournaments he entered. He lost in the semifinals last week at the Paris Masters but, since he was injured and didn't play in the event in 2012 he picked up 360 rankings points.
That gives Nadal a 12,030-10,610 lead over Djokovic. If Nadal wins two matches in round-robin play this week, he will lock up the year-end No. 1 ranking.
Djokovic made history himself by beating David Ferrer in the finals of the PNB Paribas Masters in Paris. He is the 15th player to win 40 tournaments in his career.
Eight of the Top 10 players are in London for the world Tour Finals. No. 4-ranked Andy Murray qualified but withdrew with a bank injury. Tsonga finished 235 points back of Gasquet in missing out on qualifying.
Nadal leads Group A, which also has Ferrer, Berdych and Stanislas Wawrinka. Djokovic, Juan Martin del Potro, Federer and Gasquet comprise Group B. The schedule calls for round-robin play with the two highest finishers in each group making the tournament semifinals.
The World Tour Finals finish off the ATP's 2013 calendar. The 2014 schedule opens Dec. 30 with tournaments in Chennai, India; Doha, Qatar; and Brisbane, Australia, two weeks before the Australian Open.
The ATP Top 10, listing player's name, home country and rankings points:
1. Rafael Nadal, Spain, 12,030
2. Novak Djokovic, Serbia, 10,610
3. David Ferrer, Spain, 5,800
4. Andy Murray, Great Britain, 5,790
5. Juan Martin del Potro, Argentina, 5,055
6. Tomas Berdych, Czech Republic, 3,980
7. Roger Federer, Switzerland, 3,805
8. Stanislas Warwinka, Switzerland, 3,330
9. Richard Gasquet, France, 3,300
10. Jo-Wilfried Tsonga, France, 3,065