|Murray received a medical time out at the start of the second set for a groin problem [GALLO/GETTY]
David Ferrer pulled off the first upset of the ATP World Tour Finals by beating Andy Murray 6-4, 7-5 in London on Monday.
In the opening match of Group A, Ferrer's consistency proved too much for the third-seeded Murray, who had treatment for an apparent groin injury in between sets.
The seventh-seeded Spaniard broke in the 10th game of the first set and twice came from a break down in the second to stun Murray and the home crowd inside London's O2 Arena.
Murray finished with 44 unforced errors and made just 46 percent of his first serves.
In the evening game world number one Novak Djokovic survived a match point on his way to a 3-6 6-3 7-6 defeat of Tomas Berdych in his opening group match.
The Serb, winner of three of the year's four majors, was given a thorough grilling by the hard-hitting Czech before sealing victory in two hours and 38 minutes to join Ferrer on one win in Group A.
Rafael Nadal and Roger Federer both came through three-set matches on Sunday to set up the 26th meeting of their long rivalry on Tuesday.
Murray's loss was all the more surprising given the 24-year-old Scot had won all five of his previous matches on hard courts against Ferrer and came into the tournament with a 17-1 record since losing in the US Open semi-finals to Nadal.
Murray broke for a 2-1lead in the first set but then immediately conceded the advantage with an error-strewn game. And while Murray's form fluctuated wildly, Ferrer maintained his trademark consistency from the baseline.
Murray had not shown any sign of an injury but when serving to stay in the set at 5-4 down, he asked the umpire to call for the trainer and subsequently dropped serve for the second time, blazing a forehand long and wide on Ferrer's second set point.
The trainer gave Murray a vigorous massage during the injury time-out, and it seemed to have done the trick when Murray immediately broke serve in the second set - albeit thanks to a rare error by Ferrer.
Murray's performance did not give the crowd much to shout about as he first lost serve to love in the fourth game and then double-faulted to give up another break in the eighth.
Ferrer, meanwhile, found his best form just when it counted. Leading 6-5, the Spaniard punched a perfect volley onto the sideline to bring up match point, and then pounced on an ill-advised drop shot from Murray to take control of the point and claim his first win at the O2 Arena after going 0-3 last year.
Murray will hope his body can recover quickly as he will face Berdych on Wednesday, needing a win to maintain any realistic hope of reaching the semi-finals.