Serena and Venus Williams' first doubles match together in four years ended in a first-round defeat at the Cincinnati Open, with the sisters falling 6-2,
1-6, 10-8 to Marta Kostyuk and Peyton Stearns on Monday.
The 2012 Olympic gold medallists reunited on court for the first time since the 2022 US Open, drawing a packed crowd in Cincinnati as Serena returned to competitive tennis on US soil for the first time in nearly four years.
However, the 14-time Grand Slam-winning doubles partnership could not complete its comeback, despite recovering strongly after a slow opening set.
Williams sisters fight back after slow start
Serena and Venus struggled to find their rhythm early on, with their opponents taking control to win the opening set 6-2.
The sisters responded immediately in the second set, improving their timing and shot selection as they won the first three games. Kostyuk and Stearns also dipped slightly, allowing Serena and Venus to level the match with a 6-1 second-set win.
The deciding 10-point tiebreak was far closer.
The Williams sisters fell 6-0 behind before launching a late comeback. Two double faults from Kostyuk helped them close the gap to 8-5, while Serena produced two unreturnable serves to bring the score to 8-7.
An easy volley from Serena then kept them alive at 9-8, but the comeback ended when Venus double-faulted on match point.
Kostyuk and Stearns therefore advanced with a 6-2, 1-6, 10-8 victory.
Serena gets another match ahead of US Open
The Cincinnati appearance was Serena's fourth competitive match since she began the second chapter of her career earlier this summer.
The 23-time Grand Slam singles champion had stepped away from tennis after the 2022 US Open, saying she was "evolving away" from the sport, before beginning her comeback on grass in June.
She teamed up with Victoria Mboko to win a doubles match at Queen's before playing singles at Wimbledon, where she lost to Australia's Maya Joint in the first round.
Monday's match was Serena's first on a hard court since returning.
Venus, meanwhile, has remained more active during the comeback period, playing singles and doubles in Washington, Toronto and Cincinnati after receiving wild cards. The seven-time Grand Slam singles champion had not won a match in more than a year before returning to the WTA Tour.
The sisters had been due to reunite at Wimbledon but withdrew before their opening match after Serena suffered a knee injury during her singles defeat.
US Open plans remain unclear
Serena has yet to confirm whether she will compete in singles or doubles at the US Open later this month.
The Cincinnati outing could nevertheless provide valuable match practice ahead of the year's final Grand Slam, with the US Open main draw beginning on August 31.
The sisters have won 14 Grand Slam doubles titles together and also claimed Olympic gold at London 2012.
For now, their Cincinnati campaign is over - but their latest reunion offered another indication that the Williams sisters are still willing to share the court as Serena weighs her next move in her comeback.
