Champions Daniel Nestor and Nenad Zimonjic lost all hope of defending their ATP World Tour Finals doubles title on Tuesday when they suffered their second successive round-robin defeat at the O2 Arena.
The Canadian-Serbian duo, who won the crown in Shanghai last year, once again failed to produce their best game as they went down 6-3, 6-4 to Czech Frantisek Cermak and Slovakian Michal Mertinak.
The result left the Wimbledon champions rooted to the bottom of Group A, which is topped by the undefeated pairing of Mahesh Bhupathi and Mark Knowles.
"We had high hopes coming in here. We were playing well; we were the No 1 team," Nestor said. "It's frustrating to lose to teams that we've been beating, but that's part of life."
Their defeat handed the Bryan brothers a reprieve as the Americans could now pip Nestor and Zimonjic to the year-end number one ranking.
Second seeds Bob and Mike Bryan also lost their opening match but could still qualify for the semi-finals as they have two more round-robin matches to play in Group B.
Bhupathi and Knowles relied on the greater depth of experience to outlast Polish duo Mariusz Fyrstenberg and Marcin Matkowski 3-6, 6-3, 10-7 (champions tie-break). They became the first players to book their place in Saturday's semi-finals.
"We're excited. We've played this round-robin format for many years. The easiest way to control your destiny is to win all three matches. That's been our goal. We've gotten two down and we've got one more to go," said Bahamian Knowles.