Novak Djokovic in London on Friday. (AP)

London: Novak Djokovic kept alive his chances of reaching the semi-finals of the ATP World Tour Finals by beating Rafael Nadal 7-6 (5), 6-3 Friday and handing the Spaniard his third straight loss in the tournament.

It was the third-ranked Serb’s second win of the round-robin phase, but he still needs Robin Soderling to beat Nikolay Davydenko in another Group B match to advance. Soderling has already advanced.

Meanwhile, Leander Paes and his partner Lukas Dlouhy rounded off their disappointing campaign at the season ending doubles event with their third successive defeat here Friday.

The fourth seeded Indo-Czech pair lost their last Group B match 6-7(1), 4-6 to seventh seed Max Mirnyi and Andy Ram in one hour and 15 minutes.

Nadal struggled for the third match in a row and finished with nearly three times as many unforced errors as winners - 35 to 12. He needed medical treatment on his lower back at 2-1 in the second set, and was broken in the next game.

The second-ranked Spaniard saved two match points in the final game before sending a backhand wide on the third. Djokovic had even more errors — 37 — but hit 21 winners and made fewer mistakes than Nadal when it mattered.

Nadal had broken back twice in the first set to reach the tiebreaker, where he shanked a couple of forehands and quickly fell behind 6-2. He saved three set points before sending a forehand wide on the fourth.

Nadal leaves the tournament without having won a single set, and is now on a four-match losing streak after losing to Djokovic in the semi-finals of the Paris Masters this month.

If Djokovic advances, he can overtake Nadal for the No. 2 ranking by winning the tournament.

Late on Thursday, Juan Martin del Potro squeaked into the semi-finals of the event by beating Roger Federer 6-2, 6-7 (5), 6-3 Thursday — the slimmest possible margin to advance and knock Andy Murray out of the tournament.

The result means Del Potro and Federer both advance after a three-way tie in Group A, with Murray’s elimination literally coming down to the last game after three rounds of group matches.

All three players finished the round-robin stage with two wins and identical 5-4 set records, but Murray had the lowest percentage of games won.

Had Del Potro lost just one more game against Federer, Murray would have advanced at the Argentine’s expense.

Meanwhile, Manchester City striker Carlos Tevez was present for the match and hit a few balls with Del Potro.