Five years ago, Nestor and Zimonjic beat the American twins to win the title. But the Bryans, the world's top-ranked team, needed 74 minutes to earn the victory Saturday as both Nestor and Zimonjic lost serve in the second set.
"Those guys outplayed us, we just didn't execute well enough," said Nestor, who won a title in January in Sydney with Zimonjic before losing another in Dubai in February. "Our serving was not what it should have been.
"We've played the Bryans a lot and certainly know their game. But you have to be at the top of your game to beat therm and that didn't happen for us."
The Bryans will meet Croatian Ivan Dodig and Marcelo Melo of Brazil in the final. Dodig and Melo defeated Max Mirnyi of Belarus and Russian Mikhail Youzhny 6-3, 1-6, 10-6 (match tiebreak) in their semifinal match.
Nestor and Zimonjic lost on a series of second-set errors, with the 41-year-old Canadian dropping serve and Zimonjic doing the same in the final game. Zimonjic double-faulted to set up two match points for the Americans, who cemented the victory when Zimonjic's backhand return sailed log.
Nestor and Zimonjic had seven double-faults and never had the chance at a break of their own. Instead, they lost their serve twice.
The Bryans have now won 20 straight matches. They also denied Zimonjic a shot at a sixth Monte Carlo title with four different partners, including Nestor in 2009 and 2010.