Lindsay Davenport suffered defeat in her return to competitive action in the doubles in New Haven on Wednesday.
Davenport will return to singles action next month
Former world number one Davenport, 31, and partner Lisa Raymond fell 7-6 6-3 10-4 to Cara Black and Liezel Huber.
Three-time Grand Slam winner Davenport gave birth to her first child in June and has not played in 11 months.
Jamie Murray and Jordan Kerr reached the second round of the men's doubles with a straight-sets victory over Scott Lipsky and David Martin.
Scot Murray and Australian Kerr - his third trial partner since announcing a split with Eric Butorac - won 6-4 7-5.
Having a child doesn't mean you have to abandon your career
"We were confident playing the match - and we played well in the end," said Murray.
The draw has opened up for Murray and Kerr thanks to a defeat for second seeds Jonathan Erlich and Andy Ram by Alex Kuznetsov and Ryan Sweeting.
"It's good. We get along pretty well off the court," said Kerr.
Davenport, who gave birth to her first child, Jagger Jonathan Leach, in June, plans to return to singles play next month at a tournament in Bali.
"It was fun to be back," she said. "Obviously, this is the first step in a long process.
"I feel I'm still very good at my job and having a child doesn't mean you have to abandon all your career and everything.
"I feel like while he's this young, and so far this good, I can drag him around the world and try and do what I still love to do."
In singles, Nikolay Davydenko beat American Donald Young 6-2 4-6 7-5, while Svetlana Kuznetsova held off Agnieszka Radwanska 4-6 6-1 6-4.
Hungarian teenager Agnes Szavay shocked second seed Slovakian Daniela Hantuchova 7-5 6-3 to reach the third round of the women's singles.
"She's got a great potential," said a dejected Hantuchova. "I was very surprised the way she played. I had never seen her play before. She was brave. She deserved to win."