London, Nov 17 (IANS): Andy Murray got revenge on Kei Nishikori for his defeat at the US Open but needed to battle for more than three hours to do so, gutting out a 6-7 (9-11), 6-4, 6-4 victory here in Group John McEnroe action at the ATP World Tour Finals.

Murray on Wednesday picked up his 21st consecutive victory in what was the longest match (three hours and 20 minutes) since the Finals were first played in London in 2009, boosting his hopes of finishing as year-end number 1 for the first time in his career, Efe news reported.

The top-ranked Scotsman has not yet qualified for the semifinals at tennis' eight-man, year-end championship event but can book a berth if Croatia's Marin Cilic defeats Switzerland's Stan Wawrinka on Wednesday night's round-robin action.

The Scotsman and Serbian world number 2 Novak Djokovic, each of whom are undefeated through two matches in the round-robin stage, are in a neck-and-neck battle for the top spot at season's end.

Nishikori and Murray also locked horns in an epic battle in the US Open quarterfinals in September, with the Japanese star coming out on top in five sets.

The world number 5 won the opening set Wednesday by emerging victorious in a 20-minute tiebreaker in which Murray came back from a 6-3 deficit and later saved a fourth set point before finally succumbing due to a forehand unforced error.

But Murray fought back to win the next two sets thanks in large part to the Japanese player's higher unforced-error count -- 49-39.