Andy Murray
Murray made the worst possible start to his Tour Finals campaign. Getty Images/Clive Brunskill

Andy Murray is on the brink of elimination at the ATP World Tour Finals after losing in straight sets to Kei Nishikori in London.

The US Open finalist prevailed 6-4 6-4 in a polished display on his debut at the season-ending tour finals as Murray floundered on his return to British shores.

With round robin matches against six-time Roger Federer and debutant Milos Raonic to come, neither of whom have a losing career record against Murray, the British No.1 faces an uphill task to reach a fourth semi-final.

"It's harder to qualify when you lose your first match," said Murray. "That's pretty obvious. Unlike the other events, you still have a chance to go through. If this were anywhere else, I would be out of the tournament.

"I didn't serve well enough today," added the two-time grand slam champion. "I would say that was the biggest difference in the match. When you're not serving well, obviously on your own service games you want to be looking to dictate the points."

After expressing concern regarding his reception at the O2 Arena after supporting Scottish independence in September, Murray was given a raucous welcome on his first appearance at the ATP Finals since winning Wimbledon.

In stark contrast, Nishikori was making his maiden tour finals appearance and in the early exchanges appear overawed by the occasion, producing a succession of double faults which led to Murray grabbing an early break.

The Briton's advantage was short-lived however as four straight errors allowed the Japan player back into the opening set and it was a lead Murray would be punished for relinquishing.

Nishikori did survive a fifth break point in the seventh game and the No.4 seed's resilience eventually paid off as he took the opening set as Murray dumped a poorly executed drop shot into the net.

The nerves which dominated the early moments now exclusively belonged to Murray and Nishikori stormed into a 3-0 lead early in the second set courtesy of a drop shot of his own.

Murray was now clinging on to Nishikori's coattails and saved four break points to prevent falling a double-break behind, a timely Murray drop shot getting him out of jail.

The Scot's spirit allowed him to open up a lifeline as he secured a critical break back as Murray reeled off three successive games to square the set at 4-4.

But the parity was only momentary as Nishikori first held nervelessly before a superb backhand down the line set up two match points which were converted as another Murray error began his Tour Finals campaign in wretched fashion.