Spanish pair David Marrero and Fernando Verdasco upset the top-seeded Bryan brothers to win the ATP World Tour Finals doubles title at the O2 Arena.
The sixth seeds had never even qualified for the Finals before and had lost all three of their previous matches with the world No 1 partnership but they triumphed in a superb match 7-5 6-7 (3-7) 10-7.
They saved four set points in the first set, Verdasco recovering from 0-40 down at 4-5, and the left-hander, still one of the most dangerous customers on the singles circuit, was the key man in a gripping contest.
In the next game, Mike Bryan double faulted at 15/30 to gift two break point opportunities to Marrero and Verdasco. He saved the first with a fine backhand crosscourt winner, but Verdasco clinched the game when his stinging forehand was too much for Bob Bryan, who could only volley long.
And the Spaniards clinched the next game to win the first set.
Verdasco's forehand, one of the most devastating and feared shots on the men's tour, got the Spaniards out of trouble time and time again. But the American twins - who have won the title three times before - hit back to level it up by winning the second set on the tie-break.
There was just one mini-break in the deciding Champions tie-break and it was set up by another sensational Verdasco forehand down-the-line winner.
On match point Bob Bryan could only chip a Verdasco serve long and the sixth seeds could celebrate becoming the third all-Spanish pair to win the title after Juan Gisbert and Manuel Orantes in 1975 and last year's champions Marcel Granollers and Marc Lopez.
The Bryans remain the dominant force in men's doubles though. They finished No 1 for a fifth straight year and a record ninth time overall. They have won 11 titles this season, including the Australian Open, French Open and Wimbledon.