Jamie Murray and Neal Skupski suffered a shock defeat by Liam Broady and Cameron Norrie in the opening game of Murray's Battle of the Brits.
Broady, 26, and Norrie, 24, won 3-6 7-5 11-9 as elite British men's tennis returned behind closed doors.
The six-day charity event, organised by Murray, is being played at the National Tennis Centre in Roehampton.
Later on Tuesday, Andy Murray will play his first match in seven months when he takes on Broady.
Regular doubles pairing Jamie Murray and Skupski, US Open semi-finalists last year, took the first set with ease against the singles players but Broady and Norrie recovered to win the second set and held on in the tie-break.
There are no ball kids or line judges at the events to minimise numbers on court for social distancing purposes.
"You're a really good guy Andy - Sir Andy sorry," joked Broady when asked if he had a message for Andy Murray after beating his brother Jamie.
"He's a fantastic player, a fantastic champion. He's moving really well at the moment, he's telling people he's not. We'll see how it goes. I'll save the trash talk for off camera."
The singles and doubles tournament gives top British men's players competition practice before the planned restart of the professional tour in August and will raise money for NHS Charities Together.
In the doubles, two groups of three play each other in a round-robin and the top two from each group will advance to the semi-finals.