Andy Murray has slumped to a 6-4 6-4 defeat at the hands of Kei Nishikori in his opening match at the ATP World Tour Finals.

The British number one had been opening to mark his return to the event at the O2 Arena in London with a victory but he was second-best for much of his encounter with the Japanese.

Murray broke in the fifth game of the match to seize the early advantage, but that setback sparked Nishikori into gear and the US Open runner-up dominated the remainder of the set to move ahead.

It was one-way traffic at the start of the second as Nishikori raced into a 3-0 lead, but after digging deep to avoid the double break, Murray fought back to 4-4 to give him the momentum heading into the critical part of the set.

However, as in the first set, Nishikori struck in the 10th game to break Murray's serve to seal the win on his debut at the tournament.

Murray will return on Tuesday aware that he needs to win his second match, against either Roger Federer or Milos Raonic, to give himself a chance of progression to the semi-finals.