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While Nadal's place in the semi-finals is still in doubt after his thrashing at the hands of Roger Federer on Tuesday, world number five Ferrer made sure he will be around at the weekend with a 6-3 6-1 defeat of the year's outstanding player.
In the day's other match, Czech Tomas Berdych saved a match point against Djokovic's fellow Serb Janko Tipsarevic to seal a 2-6 6-3 7-6 victory which means the calculators may be needed to decide who finishes runner-up in Group A on Friday.
The complications mean Djokovic could beat Davis Cup team mate Tipsarevic, a stand-in for injured Briton Andy Murray, and still fail to go through while a defeat, providing Ferrer beats Berdych, would see him survive.
Whatever the maths, Djokovic said he would need to play "50 percent" better in his next match.
"I have no words to explain this match," Djokovic, who went on a 41-match winning rampage at the start of the year and claimed three of the year's four majors, told reporters.
"Nothing was going well. I don't play at least 50 percent better than I did tonight, I don't think I'll have any chance. You always hope that tomorrow will bring something better."
In front of another sell-out crowd in the 17,500-capacity arena, Djokovic began solidly enough as the first six games of the match went with serve.
However, when the terrier-like Ferrer raced across court on break point to whip a forehand down the line that caught his opponent flat-footed at the net, the match ceased to become a real contest as the 29-year-old from Valencia ran way with it.
He broke the Djokovic serve again to win the opening set and he needed nothing spectacular in the second set as Djokovic offered little resistance. The Serb's mounting unforced error tally spoke volumes.
"I'm embarrassed to look at the stats," Djokovic, who made 33 unforced errors, said.
"All credit to my opponent, he played a great match but, you know, I wasn't there."
World number nine Tipsarevic, one of the year's most improved players, can no longer reach the semi-finals after his defeat by Berdych although he can content himself with his $70,000 fee for being an alternate and the $120 000 participation fee after replacing the injured Andy Murray.
He should also have banked another $120 000 for a round-robin victory but a skewed volley on match point in the deciding set tiebreaker proved extremely costly.
Berdych looked set for a second consecutive defeat, after his three-set loss to Djokovic on Monday, when Tipsarevic turned the breaker round to lead 6-5 on serve.
He struck lucky though as Tipsarevic let him off the hook.
First the bearded Serb's backhand volley went inches wide of the sideline on match point.
He followed that with a crazy double fault and the match ended painfully when he stumbled over trying to return a Berdych serve. In trying to get to his feet he fell again and ended up in a heap as a relieved Berdych played the ball away into an empty court.
Tipsarevic hobbled off after whacking his knee on the court although he said he would be okay to play Djokovic.
"If I have gotten the chance one more time, I would still take that ball early and go to the net and play the volley," said the Serb who began the year ranked 49th in the world.
"I am not honestly at all disappointed because of the way I played on the match point.
"My feeling now is that I will be ready for practice tomorrow afternoon."
Berdych was just happy to still be in with a shout.
"I was a bit unlucky on Monday but I got luckier here," he said.
"There is still a good chance to qualify and go through. So we will see."
In Group B Federer has already guaranteed a top-two place and he will be joined in the semis by the winner of Nadal and Jo-Wilfried Tsonga who face off on Thursday evening.
Frenchman Tsonga is expecting a backlash from Nadal after his humiliating 6-3 6-0 defeat by Federer.
"Rafa is a champion, he has pride," Tsonga told Reuters. "Such a defeat will be stuck in his throat. He's hit but not sunk. It's like when you hunt. If you miss the boar, you just run away."
