Andy Murray suffered the worst possible start to his ATP World Tour Finals campaign with a straight sets defeat by David Ferrer of Spain.
It was a bitterly disappointing performance from the British number one, who has been tipped by many to reach the final but may now struggle to advance past the group stage.
Murray required treatment for an injury after losing the first set and he became increasingly angry with himself as he twice saw breaks of serve overturned in the second.
The impressive Ferrer came through 6-4 7-5 in exactly two hours in front of a stunned crowd at London's O2 Arena, Murray finishing with 44 unforced errors.
It means the Scot's second round-robin match on Wednesday will be against the loser of Monday's 2000 GMT encounter between Novak Djokovic and Tomas Berdych.
Murray came into this match as the firm favourite, with a 17-1 win-loss ratio since the US Open and victory from each of his five previous meetings with Ferrer on hard courts.
But at his pre-tournament press conference the world number three revealed he was nursing "a few niggles" from the Paris Masters and his movement seemed far from smooth in the early stages.
A string of Ferrer errors enabled Murray to break serve in game three, but he immediately relinquished the advantage with a wayward forehand after one of several poor drop shots.
The 2007 runner-up Ferrer was nullifying his more illustrious opponent from the baseline and it made for a terribly flat atmosphere inside a stadium featuring vast swathes of empty seats.
The crowd were finally brought to life when Murray produced an astonishing crosscourt backhand pass in game six, followed by a belting forehand winner as he held for 3-3.
But the right-hander from Dunblane could not breach Ferrer's incredible defences and the resilient world number five saved a break to go 5-4 ahead before picking the perfect moment to strike.
With his first-serve percentage down at a paltry 37%, Murray was getting extremely frustrated and after failing to reach a drop shot, he shocked a ball boy with two loud shouts.
Ferrer took his second set point when his opposite number shanked a forehand, and at the changeover the trainer was called to massage Murray's left leg.
There was little sign of discomfort at the start of a topsy-turvy second set, Murray racing into a 2-0 lead and seemingly in control of Ferrer.
However, a dreadful game four saw him broken to love and although he moved ahead again in game eight, an untimely double-fault levelled things up at 4-4.
Murray was plagued by inconsistency and repeatedly shouting in the direction of his support team, and a scare in game 10 was the precursor to his ultimate fate.
Ferrer planted a stunning volley on the line to bring up match point, and he took it by sweeping home a crosscourt backhand winner.
In the afternoon doubles match, sixth seeds Robert Lindstedt and Horia Tecau beat Indian pair Mahesh Bhupathi and Leander Paes 7-6 (8/6) 6-1.