It’s just the round-robin stage, but Rafael Nadal can already lock down the world number one spot in the ATP end-of-year rankings if he can clinch one of the final four slots of the 2013 APT World Tour Finals on Wednesday afternoon at the O2 in London, England.

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Nadal can qualify for the semis—and consequently, be the top-ranked player in tennis for the third time—in two scenarios; first, if he can beat Wawrinka in straight sets, or if he gets to win and compatriot David Ferrer beats Tomas Berdych in the second singles match of the day.

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Wawrinka defeated Berdych, 6-3, 6-7 (0), 6-3, on Monday and will qualify for the semifinals if he can pull off the upset against Nadal and Berdych defeats Ferrer.

Novak Djokovic is still mathematically in the race for the top spot in the year-end rankings, but the Serbian has to pray that Nadal does not make the semis, very improbable at this point after the Spaniard won big against Ferrer, 6-3, 6-2, in his opening match in the final ATP tournament of the season.

Nadal has 12,030 points to Djokovic's 10,610, entering the tournament in London.

Nadal is a perfect 11-0 against Wawrinka in their head-to-head match-ups since the 2007 Australian Open. The most recent Nadal victory (7-6 (10), 6-1) is at the quarterfinals of the ATP World Tour Masters in Shanghai, China on October.

Curiously, Nadal is also a perfect 24-0 in sets played against Wawrinka— confirming the Spaniard’s domination of the Swiss in all of their matches.

Wawrinka knows of the difficult task ahead if he wants to avoid a 12th straight loss to Nadal.

"For sure he's a much better player than me. He's No. 1 in the world. He's had an incredible year. It doesn't matter if I play him indoors or outdoors, it's going to be a tough match for me," said Wawrinka, who is playing in his first-ever ATP World Tour Finals, to the official Web site of the tour.

Meanwhile, Ferrer holds a 7-3 edge in 10 career meetings with Berdych. Both need a win in this match-up for a chance to advance to the next round.

The ATP World Tour Finals has a round-robin format which means that each player is assured of at least three matches. The players take on their groupmates once, and the top two players in each group advance to the knockout semifinal round.

Nadal, Ferrer, Berdych and Wawrinka are in Group A, while Djokovic, Roger Federer, Juan Martin Del Potro and Richard Gasquet are in Group B.

2013 ATP World Tour Finals Round Robin Live Streaming and Schedule

Nov 7, 2013 - AEDT
1:00 a.m Rafael Nadal (Spain) vs. Stanislas Wawrinka (Switzerland)
7:00 a.m. David Ferrer (Spain) vs. Tomas Berdych (Czech Republic)

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