
Nalbandian, a former number three who dropped to his current 25th through knee and back problems during the first half of 2007, hammered Spain's world number two for the second time in two weeks.
The result, after a 6-1, 6-2 humiliation administered a fortnight ago in the Madrid quarter-finals, left Nadal 25-1 in Paris after three straight trophies at Roland Garros and his first Bercy appearance.
Nalbandian's recent long-odds run of form has vaulted him into the second alternate's position for the elite eight-man Masters Cup starting next Sunday in Shanghai.
In 2005, the Argentine won the event after coming on board late as a substitute.
"I was injured (knee and back) the first part of the year, so it wasn't very easy for me to come back and play good tennis," said the South American.
"After US Open I thought I was playing much better. I was healthy physically and mentally, too.
"I thought I would have better results than in the first part of the year, but I never thought I could win Madrid and Paris."
Nalbandian has now won two major indoor titles this autumn by beating both Nadal and world number one Roger Federer twice apiece.
He stopped the Swiss in the Madrid final and took him out in straight sets at the Bercy third round.
Nadal, who won just 18 per cent of points on second serve, refused to go into major depression.
With his trip to Shanghai just a day or so away, the Spaniard, who has suffered form knee tendinitis, was accentuating the positive.
"It's not a big disappointment. I lost so I'm not very happy right now. But I know I'm playing a very good tennis. I'm not disappointed, I'm very happy."
Nalbandian went without a coach for much of the early part of the season, but began turning his game around one he took on fellow Argentine Martin Jaite.
His win leaves him 2-0 over Nadal.
"I played all week, now just today," said the Argentine. "I don't know why this result was so easy.
"In the beginning, it was a little tougher for me. Maybe I was a little nervous in the first few games.
"But after that everything felt great on court, great feeling on the ball, and played amazing."
Nalbandian said he will decide later whether to travel to Shanghai to stand by for the year-end spectacular.
Those into the field include Federer, Nadal, Novak Djokovic, Nikolay Davydenko, Andy Roddick, David Ferrer, Fernando Gonzalez and Frenchman Richard Gasquet who achieved last-gasp qualification on Saturday. dpa