World number one Novak Djokovic was beaten for only the fifth time this year as Spain's David Ferrer booked his place in the semi-finals of the ATP World Tour Finals with a 6-3, 6-1 victory on Wednesday.
Djokovic had won 70 of his previous 74 matches during a majestic campaign, but the Serb was completely out-played by fifth seed Ferrer, who joins Roger Federer in the last four at London's O2 Arena.
Although Djokovic has enjoyed one of the greatest single seasons in tennis history after winning three Grand Slam titles and five Masters crowns, he has been flagging in recent weeks.
The 24-year-old has been hampered by back and shoulder injuries since winning the US Open and was forced to withdraw from the Paris Masters.
On the evidence of this lacklustre effort, the Tour Finals might be a tournament too far for Djokovic, although he can still qualify for the semi-finals with a win over compatriot Janko Tipsarevic on Friday.
"I have no words to explain it," Djokovic said. "It was the worst match I have played all year but all credit to David. He played really well."
Ferrer added: "I think maybe he was a little tired but I played very well and didn't make any mistakes.
"It is important for me to play well here. Last year I didn't win and now I have won twice, so I am very happy."
Both players walked on court knowing they could seal their place in the last four with a victory, although Ferrer needed to work slightly harder than his opponent as only a straight-sets triumph would be enough, while Djokovic could afford to win by any margin and still go through.
However, Djokovic was below his best against Tomas Berdych in his opening Group A match on Monday -- he had to save a match point before eventually coming through in the final set tie-breaker -- and Ferrer was able to take advantage of another tired display.
Djokovic squandered an early chance to seize the momentum when he lofted a forehand well long on break point in the fourth game and Ferrer was able to scramble a service hold from a tricky position.
Ferrer had given a ruthless performance against the ailing Andy Murray in his straight sets win on Monday and the Spaniard was in obdurate mood again.
While Ferrer is unlikely to ever attract the headlines garnered by Djokovic, Roger Federer and company, the world number five has quietly become a regular presence in the latter stages of Grand Slams and in the upper echelons of the rankings.
His remarkable consistency from the baseline makes him a tough nut to crack as Djokovic discovered when the 29-year-old produced a fizzing forehand to break for a 4-3 lead.
Ferrer's clever shot selection was keeping Djokovic was establishing any rhythm and the Serb shook his head forlornly as another unforced error gave away a second break to end the first set.
Djokovic was under pressure again in the second game of the second set. He saved two break points but couldn't stave off a third as Ferrer unloaded a fierce winner from the baseline.
Ferrer was just too solid in every aspect of the game for Djokovic to harbour much hope of a comeback and the Spaniard underlined his superiority with one more break before serving out a win almost as impressive as Federer's demolition of Rafael Nadal on Tuesday.