Daniel Nestor of Toronto and Serbia's Nenad Zimonjic were upset in their opening match at the 2009 Barclays ATP World Tour Finals on Sunday.
Nestor and Zimonjic, top seeds and defending champions, fell 6-4, 6-4 to Poland's Mariusz Fyrstenberg and Marcin Matkowski in their men's doubles tilt.
"Today our serve worked almost perfectly," Matkowski said on the ATP website. "We didn't face a single break point. I guess Daniel and Nenad were under pressure, they had to hold their serve. That was a very good match for us, the best we played all year. If we play like this, we should be very good in this tournament."
Nestor and Zimonjic, who entered the tournament with consecutive titles at the Davidoff Swiss Indoors Basel and the BNP Paribas Masters, are scheduled to resume round-robin play on Tuesday.
Eight teams are divided into Group A and Group B with the top four teams advancing to the semifinals.