Rafael Nadal retained his Monte Carlo Masters title with a 6-2 6-7 (2-7) 6-3 7-6 (7-5) win against Roger Federer.
Nadal's clay form is already looking ominous for his French Open rivals
The Spanish second seed took the first set after racing to a 5-1 lead, but Federer found his form in the second.
Nadal still broke him to love in the seventh game before his nerve wobbled, allowing the Swiss star to break back for 5-5 and then win the tiebreaker.
But Nadal showed why he is the world's best on clay in the third set, then edged a tense fourth to seal victory.
Federer looked to have extended an epic final into a fifth set when he took a 3-0 lead in the second tiebreaker.
But the 19-year-old Spaniard fought back as the shadows lengthened on court at the Monte Carlo Country Club.
A forehand winner on his first match point finally sealed victory and his 14th career title after three hours and 49 minutes of thrilling tennis.
"It was a very unbelievable day for me," said Nadal. "It's special to begin the clay season like this.
"Beating Roger in the final is even more special, it's great."
Federer, who had never progressed past the quarter-finals in Monaco before this year, admitted: "Rafael played a great match, it was tough to lose."
Nadal had also inflicted the world number one's only other defeat of 2006, beating him in the final of the hardcourt Dubai Open on 4 March.
Federer also lost to Nadal in the semi-finals of last year's French Open, the only grand slam tournament he has yet to win.
"I played a smarter match today than last year's Paris semi-final," Federer added. "I have moved closer to him on clay and that's very important."
Nadal now has a 4-1 record against the Swiss world number one, and has chalked up a remarkable 42 successive wins on clay.
The left-hander is third on the all-time list behind Guillermo Vilas (53) and Bjorn Borg (46).