
The all-Iberian clash which lasted for well over two and a half hours means that winner and runner-up in the round-robin will be decided in final matches on Thursday.
Ferrer, with two wins, sits atop the group with Nadal provisional second.
"It's one of the tougher tournaments of the year," said Nadal. "We play against the best always, every match, so anything can happen."
Richard Gasquet sent Serb Novak Djokovic to certain elimination with a resounding 6-4, 6-2 victory.
The Red group resumes on Wednesday with Roger Federer facing a first-ever must-win match at the year-end championships after he lost in a shocker on Monday to Chile's Fernando Gonzalez.
The Swiss world number one, who was officially awarded year-end honours for 2007 in an on-court ceremony, plays Nikolay Davydenko, against whom he's never lost.
"This year was very special because I was behind Nadal (in year-end race points) and won it back," said Federer. "It's the fourth consecutive year for me to finish as number one.
"It's an amazing run I'm on and I'll try my best to maintain it in 2008."
Outsider Ferrer proved that he had no pressure as he beat Spanish powerhouse Nadal for the second straight time after a US Open fourth- round upset.
Ferrer clawed back after dropping the opening set and came through to level the contest after Nadal saved two set points in the second.
In the third, the three-time French Open winner Nadal went down a double break 1-5, fighting back to 3-5 and saving a match point in the eighth game.
Ferrer stiffened in the final game, serving out the win with a backhand winner cross-court off a failed Nadal drop shot.
Ferrer overcame 38 unforced errors in a patchy, but successful, effort.
"He's playing with unbelievable confidence, moving unbelievable, his playing is crazy," said Nadal. "I congratulate him.
"I didn't play a bad match, I fought a lot, but I couldn't win."
In the early match, Gasquet's record improved to 1-1 on the year through a win for the last man to qualify into the field, while Djokovic stands nil for two this week.
"I played well, I was confident," said Gasquet. "I had nothing to lose against Novak because he's three in the world.
"I just had to enjoy the moment and play my game. I had a perfect match, amazing shots with my backhand.
"It was a really good match for me."
Djokovic, worn out after a long season of 68 wins and 18 losses, was unable to make an impression on the Frenchman.
"I didn't find any solutions, he was playing well," said the subdued Serb. "He was very patient.
"But i I don't want to look on the negative side. I had an amazing season - but it was too long for me.
"I couldn't be even close to my level. I just couldn't be so confident on the court."
By Bill Scott, dpa