It is all or nothing now as four of the world’s Top 5 in the Emirates ATP Rankings square off for a place in Sunday’s championship match at the Barclays ATP World Tour Finals. The greatest rivalry in the men’s game is renewed when World No. 1 Novak Djokovic and former No. 1 Rafael Nadal do battle, while the two greatest Swiss men’s players in history – Roger Federer and Stan Wawrinka – meet again in blockbuster semi-final showdowns.

Four-time champion Djokovic faces No. 5 seed Nadal for the 46th time, the most meetings between two players during the Open Era. Nadal carries a narrow 23-22 FedEx Head2Head lead into the clash, but Djokovic could tie the series for the first time. The Spaniard has lost seven of his past eight meetings against the Serb. Three times they have met in 2015, three times Djokovic has prevailed in straight sets – in the semi-finals at ATP Masters 1000 Monte-Carlo, the quarter-finals at Roland Garros and in the Beijing final.

The 28-year-old Belgrade native beat Kei Nishikori 6-1, 6-1 on Sunday and Tomas Berdych 6-3, 7-5 on Thursday. In between, he fell to Federer 7-5, 6-2 on Tuesday. His loss broke streaks of 38 consecutive indoor match wins, 23 overall this season and 15 straight at the Barclays ATP World Tour Finals. The 29-year-old Spaniard opened on Monday with a 6-3, 6-2 win over Wawrinka, followed by a 6-4, 6-1 victory over Andy Murray on Wednesday, before beating No. 7 David Ferrer 6-7(2), 6-3, 6-4 on Friday to qualify for his fifth semi-final in the season finale.

The winner will face either six-time champion Federer or No. 4 seed Wawrinka. Federer and Wawrinka battled for two hours and 48 minutes at The O2 in the 2014 semi-finals, with Wawrinka holding four match points in the final set, before Federer emerged with a 4-6 7-5 7-6(6) triumph. A week later the pair combined to deliver Switzerland its first Davis Cup title.

By winning all three of his round-robin matches, Federer has guaranteed he will finish as the Swiss No. 1 for the 15th straight season in the Emirates ATP Rankings. But the gap between third-ranked Federer and fourth-ranked Wawrinka is shrinking by the year. The Olympic gold medalists and Davis Cup champions met three times this year – Federer posting wins in the ATP Masters 1000 Rome semi-finals and US Open semi-finals and Wawrinka winning in straight sets on his way to the title at Roland Garros in between.

The 34-year-old Federer beat Berdych 6-4, 6-2 on Sunday and followed with the win over Djokovic before registering a 7-5, 4-6, 6-4 win over Nishikori to reach the semi-finals for a record 13^th time in 14 years. The 30-year-old Wawrinka opened on Monday with the previously mentioned loss to Nadal before bouncing back with a 7-5, 6-2 win over Ferrer on Wednesday and a 7-6(4), 6-4 win over Murray on Friday. He has reached his third consecutive semi-final at the season-ending championship.

In doubles, the race for year-end No. 1 in the Emirates ATP Doubles Team Rankings is nearing a dramatic finish. Jean-Julien Rojer and Horia Tecau lead Bob and Mike Bryan by 45 points and Ivan Dodig and Marcelo Melo by 370 points. But each team can claim year-end No. 1 by winning the title on Sunday. Rojer and Tecau are 9-0 in London this season after winning the Wimbledon title and going undefeated in round-robin play. 

The Bryans saved five match points against Jamie Murray and John Peers on Thursday to keep hopes alive for their seventh straight No. 1 finish (11 in the past 13 years). Dodig and Melo are 12-1 this season in match tie-breaks, including 2-0 this week at The O2. Also in the semi-finals are Rohan Bopanna and Florin Mergea, who did not qualify until the last day of the regular season on Nov. 8.

STREAKY SERIES BETWEEN DJOKOVIC AND NADAL
For the fourth time at The O2 and fifth time at the year-end championships overall, Novak Djokovic and Rafael Nadal will face off. Nadal leads the back-and-forth series 23-22 (61-60 in sets), but the Spaniard has lost seven of his past eight meetings against the Serb. It’s not the first time that one player has dominated the other.

RIVALRY FOR THE AGES
Novak Djokovic and Rafael Nadal (46) have met more than any other two players in the Open Era. Djokovic and Federer are a close second with 43 meetings.

WAWRINKA CLOSING THE GAP
By winning all three of his round-robin matches, Roger Federer has guaranteed that he will finish as the Swiss No. 1 for the 15th straight season in the Emirates ATP Rankings. But the gap between third-ranked Federer and fourth-ranked Stan Wawrinka is shrinking by the year. Going back to the 2013 US Open, Wawrinka has been the better Swiss at the Grand Slams.

* Received 1 walkover