British number three Cameron Norrie retired from his Battle of the Brits match against Dan Evans through injury.
Norrie, 24, trailed 4-0 in the first set before retiring with a hip problem at the National Tennis Centre.
Norrie, ranked 77th, decided not to risk making it worse with the professional tours resuming this month.
Evans' win helped the Union Jacks team cut the gap on the British Bulldogs to 43-40 on the penultimate day of the week-long event in Roehampton.
Earlier on Saturday, Katie Boulter, Jan Choinski and Jodie Burrage all won their singles matches for the Jacks.
The Bulldogs, who led 41-32 overnight, earned their only win of the day so far through former world number 14 Kyle Edmund.
The country's leading men and women are playing together in singles, doubles and mixed doubles matches in a week-long competition.
It has been organised by Jamie Murray to provide competitive action for British players while the professional tours remain suspended because of the coronavirus pandemic.
The first team to 60 points will win the event, which features 40 singles and 20 doubles matches worth varying points and finishes on Sunday.
Saturday's order of play/results
Union Jacks 40-43 British Bulldogs (first to 60)
(Matches show Union Jacks players first)
Katie Boulter 6-1 6-1 Beth Grey (2pts)
Anton Matusevich 2-6 1-6 Kyle Edmund (2pts)
Dan Evans 4-0 ret. Cameron Norrie (3pts)
Alicia Barnett/Olivia Nicholls v Harriet Dart/ Beth Grey (2pts)
Not before 16:30 BST:
Andy Murray/Lloyd Glasspool v Joe Salisbury/Kyle Edmund (2pts)
Jamie Murray/Heather Watson v Liam Broady/Harriet Dart (3pts)
Jan Choinski 6-4 4-6 10-4 Alastair Gray (1pt)
Jodie Burrage 2-6 6-2 10-7 Emma Raducanu (2pts)