Spain's David Ferrer stunned home favourite Andy Murray in straight sets in their opening Group A match at the ATP World Tour Finals in London.
The world No.5 capitalised on an error-strewn display from Murray, which included no less than 25 unforced mistakes on the backhand-side, to record a 6-4 7-5 victory.
With the win Ferrer, who lost all three of his round robin matches at the same venue in 2010, gained revenge for two recent defeats to Murray in Tokyo and Shanghai as well as recording his first ever hardcourt triumph over the Scot.
Murray will now have to beat world number one Novak Djokovic and Tomas Berdych to keep alive any hopes he has of progressing from Group A.
Predictable
Diminutive Ferrer has had an impressive season, establishing himself as the leading player outside the top four, but Murray had won their previous four meetings and never lost to his opening opponent away from the clay courts.
The match quickly adopted a predictable pattern, with Ferrer drilling shots from the baseline while Murray attempted to dictate the tempo.
Murray got his reward with a break in only the third game, finally taking his fourth chance, but he promptly gave the advantage straight back with a loose series of points.
Murray has been in great form this autumn, winning titles in Bangkok, Tokyo and Shanghai, but he was struggling to get going in front of his home fans.
There were glimpses of that form in the sixth game, not least a searing backhand pass that clinched a quite brilliant point - much to the delight of an O2 Arena that was, disappointingly, far from full.
The Scot, watched by good friend David Haye, was making life difficult for himself with a poor first-serve percentage and, when he did get a chance to break in the ninth game, he was made to pay for being too passive.
Murray appeared to be struggling with a leg problem and he was certainly not moving particularly well.
Suddenly Ferrer, who was playing a very solid match, was threatening the Murray serve to take the set. One chance disappeared with a wayward forehand but on the second it was the home hope who miscued.
Fascinating
The trainer arrived to see Murray before the start of the second set, massaging and stretching out the world number three's left leg and hip.
Ferrer, the most dogged of Spanish baseliners, is certainly not the player one would want to face when struggling physically but Murray received an immediate boost with a break in the opening game after his opponent netted the simplest of backhands.
Again the 24-year-old's break did not last long, with Ferrer winning four successive points on his opponent's serve to level at 2-2, and Murray looked in real trouble when a mis-hit and net cord helped the Spaniard to two more break points in the sixth game, but this time he held on.
In the next game, Murray raised the pace to move a break ahead once more, only to give it straight back with a double fault.
It was a fascinating contest, if far from a great match, but Murray's weakness on serve eventually proved the decisive factor as Ferrer brought up a first match point at 5-6 and took it with a backhand winner.