While Rafael Nadal sits comfortably atop the ATP rankings, the No. 2 spot is on somewhat shaky ground.

After capturing the Wimbledon and U.S. Open titles, Novak Djokovic has surged in the rankings. He is currently No. 3 but only trails No. 2 Roger Federer by 455 points. 

It has been a major comeback for the Serbian star. In May, Djokovic was knocked out of the Madrid Open in the second round by Kyle Edmund. The loss meant the 31-year-old would fall out of the top 20 for the first time since 2006.

Djokovic would later advance to the quarterfinals of the French Open, then the finals of the Queen's Club Championships in London before winning Wimbledon. After a Round-of-16 setback at the Rogers Cup, Djokovic would defeat Federer in straight sets in Cincinnati and then proceed to not drop a set in five straight U.S. Open matches, including the final against Juan Martin del Potro.

Former tennis star Greg Rusedski recently said that he thinks Djokovic will surpass Nadal to be the world No. 1 by the end of the year.

There has been a heated competition among the big three for rankings and Grand Slam titles. Federer has won 20 Grand Slam titles, Nadal has won 17 and Djokovic has won 14. 

In 2018, with the four Grand Slams complete, Nadal has a 45-4 record, while Federer, who turned 37 in August, has a 36-6 record. Djokovic has a 40-10 record. 

Del Potro has also climbed in the rankings. After advancing to the U.S. Open final, the hard-hitting Argentine has moved up to No. 4. in the world, jumping ahead of 21-year-old Alexander Zverev of Germany.

Rounding out the top 10 are No. 6 Marin Cilic, followed by No. 7 Grigor Dimitrov, No. 8 Dominic Thiem and No. 9 Kevin Anderson.

American John Isner, who in July reached No. 8 in the world, is currently ranked No. 10. Other Americans of note include Jack Sock (No. 17), Steve Johnson (No. 30), Frances Tiafoe (No. 40), Ryan Harrison (No. 51), Tennys Sandgren (No. 58) and Sam Querrey (No. 61). 

ATP Rankings (Points)

1. Rafael Nadal - 8,760 

2. Roger Federer - 6,900

3. Novak Djokovic - 6,445

4. Juan Martin del Potro - 5,980

5. Alexander Zverev - 4,890

6. Marin Cilic - 4,715

7. Grigror Dimitrov - 3,755

8. Dominic Thiem - 3,665

9. Kevin Anderson - 3,595

10. John Isner - 3,470

GettyImages-1030136428 (1) Novak Djokovic came out on top at the 2018 U.S. Open. Photo: Chris Trotman/Getty Images for USTA