LONDON    -    Rafael Nadal has already clinched year-end No. 1 in the ATP Rankings, but he wants more at the Nitto ATP Finals. The Spaniard prevailed in a 6-7(4), 6-4, 7-5 marathon on Friday against Stefanos Tsitsipas in Group Andre Agassi.

Nadal needed to defeat Tsitsipas in order to maintain his chance at advancing out of round-robin action. Daniil Medvedev must now beat Alexander Zverev in the night session in order for the Spaniard to secure his place in the semi-finals. The World No. 1 is trying to reach the last four at the season-ending championships for the first time since 2015 and sixth time overall. He is 5-2 in final round-robin matches at this event.

The Spaniard secured year-end No. 1 in the ATP Rankings after Roger Federer defeated Novak Djokovic on Thursday. It’s the fifth time that Nadal has finished a season in the top spot. Nadal improved to 5-1 in his FedEx ATP Head2Head rivalry with Tsitsipas. All three of their previous clashes this year took place in the semi-finals. Nadal prevailed at the Australian Open and Internazionali BNL d’Italia, while Tsitsipas scored an upset at the Mutua Madrid Open.

Tsitsipas qualified for the semi-finals on Wednesday by winning his first two matches in Group Andre Agassi. The Greek is making his debut at The O2. Neither player faced a break point in the opening set. Nadal and Tsitsipas remained even until 4/4 in the tie-break, when the Greek secured a mini-break after Nadal hit a backhand into the net. Tsitsipas closed out the set with an ace two points later. He finished the set with 13 winners and 13 unforced errors.

But Nadal wasn’t ready for his time at The O2 to end. He earned the first pair of break points of the match at 2-2 in the second set, but couldn’t convert. A third break point came and went at 3-3 after a strong first serve from Tsitsipas. But Nadal made good at 4-4 and roared in approval after Tsitsipas launched a forehand wide. The Spaniard comfortably held in the next game to even the score. Nadal produced 11 winners to just six unforced errors in the second set and lost five points on serve (20/25).

It appeared that Nadal might run away with the match after knocking off a backhand volley to grab two break points at 2-2 in the deciding set, but Tsitsipas bravely fought back with aggressive play. The Greek then saved another break point at 3-3 with a big first serve. Both men saved their their best tennis for the final stages of the match and traded high-quality winners with each other.

But after nearly three hours of play, Nadal finally secured the break at 5-5 after Tsitsipas pushed a backhand volley wide. The World No. 1 raised his arms in triumph after holding serve in the next game to prevail after two hours and 52 minutes. Nadal finished the night with 38 winners to 30 unforced errors and didn’t offer his opponent a break point.

Earlier, Roger Federer lit up the O2 Arena with a dazzling 6-4 6-3 defeat of Novak Djokovic to hand the Serb an early ATP Finals exit and wreck his bid to end the year ranked number one. In the 49th meeting between the two great rivals, Federer snapped a five-match losing run against Djokovic and gained some consolation for his heartbreaking Wimbledon final defeat. After both men had been beaten by Dominic Thiem earlier in the group, their eagerly-anticipated duel was a straight shoot-out to join the Austrian in the semi-finals.

Cheered on by the majority of the 17,000 fans in the arena, Federer produced an astonishing display of power, grace and precision and Djokovic simply had no answer. Djokovic’s first defeat by Federer for four years means Rafael Nadal will end the year as world number one for the fifth time even if the Spaniard fails to reach the semi-finals. Federer, Djokovic and Nadal have all achieved the feat five times. Pete Sampras managed it a record six times.