With upsets against higher-ranked players in the group stages, Switzerland’s top two players Roger Federer and Stanislas Wawrinka are into the final four of the 2013 ATP World Tour Finals to take on the top two players in the world in Rafael Nadal and Novak Djokovic, respectively at The O2 in London.

Wawrinka defeated Spain's David Ferrer (6-7, 6-4, 6-1) and Czech Republic's Tomas Berdcyh (6-3, 6-7 (0), 6-3) in Group A while Federer beat Argentina's Juan Martin Del Potro (4-6, 7-6 (2), 7-5) and France's Richard Gasquet (6-4, 6-3) in Group B.

For their part, Nadal and Djokovic showed their top form in sweeping the round-robin stage. Nadal had wins over Wawrinka (7-6 (5), 7-6 (6), Berdych (6-4, 1-6, 6-3) and Ferrer (6-3, 6-2) while Djokovic with victories against Federer (6-4, 7-6 (2), 6-2), Del Potro (6-3, 3-6, 6-3) and Gasquet (7-6 (4), 4-6, 6-3).

The crossover semifinals from the top two players in each group pairs Wawrinka against Djokovic and Federer against long-time rival Nadal.

Federer is appearing in his 11th semis appearance in the ATP season finale and is the oldest player to reach the final four since Andre Agassi appeared in 2003.

Nadal and Federer has met 31 times in their careers and the 27-year-old Spaniard has dominated the former world number one winning 21 of these matches. Nadal has also won all three matches in three match-ups in 2013 including the finals at the ATP World Tour Masters in Italy, Rome.

Meanwhile, Wawrinka goes in to the semis as a big underdog as well. He only has won 2 of 14 matches against Djokovic, who has won the last 13 matches in this head-to-head. The last time Wawrinka won against Djokovic was way back in 2006.

However, Wawrinka has indulged Djokovic in two of the most memorable and closely-fought contests in 2013.

In the semifinals of the 2013 U.S. Open, Djokovic needed 5 sets against Wawrinka with a score of 2-6, 7-6 (4), 3-6, 6-3, 6-4. In the Round of 16 of the Australian Open, they also went the full route and the match ended when Djokovic won the deciding fifth set via tiebreak, 12-10.

2013 ATP World Tour Finals
Semifinals Schedule (AEDT)

1:00 a.m. Rafael Nadal (Spain) vs. Roger Federer (Switzerland)
7:00 a.m. Novak Djokovic (Serbia) vs. Stanislas Wawrinka (Switzerland)