By Martyn Herman

LONDON (Reuters) - Andy Murray and Novak Djokovic are careering towards a Sunday showdown to decide their year-long battle for supremacy.

The two 29-year-olds, rivals since their junior days, have matched each other stride for stride so far at the ATP World Tour Finals with Murray recording his second group win on Wednesday after a three-hour battle with Kei Nishikori.

Djokovic, usurped as world number one last week by Murray after 122 weeks at the summit, has already won his group after defeating Dominic Thiem and Milos Raonic, even before Thursday's match against alternate David Goffin.

Murray is favorite to finish top of his group after claiming the scalps of Marin Cilic and Nishikori.

Few would bet against them contesting Sunday's final and the TV executives will be positively praying for it.

"I mean, I think for the tournament, for everyone interested in tennis, that would probably be the perfect way to finish the year," Murray, whose incredible 21-match winning streak has propelled him top of the rankings, told reporters.

"For me and I'm sure for Novak, both of our goals would be to try to win the event. For him, whether that's by beating me, someone else, or for me if it's winning against Novak or another player, it doesn't change for us as players.

"But (to play against each other in the final) would be the most exciting way to finish the year."

Not since 2001 has the number one ranking changed in the final week of the season, when number two Lleyton Hewitt dethroned number one Gustavo Kuerten in Sydney.

Should Djokovic, winner of the event for the past four years, and Murray meet on Sunday it what would be their first clash since Djokovic's French Open triumph in June.

Quite what shape Murray would be in though is unclear.

He has endured an exhausting schedule in the autumn, winning four consecutive titles and used up precious reserves of energy in the three hours and 20 minutes it took him to subdue Nishikori 6-7(9) 6-4 6-4 on Wednesday -- the longest match since London began hosting the tournament in 2009.

Murray was in the ice bath immediately afterwards and his powers of recovery will be put to the test over the next few days with Wawrinka waiting on Friday followed by a likely semi-final with big-hitting Canadian Milos Raonic.

"It could come down to a match between me and Novak," Murray said. "Who knows what's going to happen the next few days. Just from my side I'm concentrating on trying to win my own matches.

"Make it as tough as possible for Novak to jump me."

Murray's win on Wednesday means he has 11,585 points to Djokovic's 11,180 but with 275 Davis Cup points to come off the Briton's total there is still everything to play for.

(Reporting by Martyn Herman; Editing by Toby Davis)