British number one Johanna Konta was beaten in her first match since March as Jodie Burrage caused a shock at the Battle of the Brits on Monday.
Konta, ranked 14th in the world, was understandably far from her best and lacked fluency in a 6-4 6-3 defeat.
Burrage, 21, has reached the final in two LTA tournaments recently and looked sharp in a confident performance.
"It feels amazing to win against such a great player, I feel overwhelmed," said the British number seven.
"I was surprised with how I played. When I found out I was playing her I was nervous and didn't want to get hit off the court.
"This morning I was excited to get out there and swing. I didn't have any pressure."
Burrage's win gave her Union Jacks side an early lead against the British Bulldogs in a unique team event featuring the country's leading men and women at Roehampton.
British men's number one Dan Evans lost 4-6 6-3 10-7 in his opener against Cameron Norrie, while there were also singles wins for Kyle Edmund, Heather Watson and Aidan McHugh.
The event has been organised by Jamie Murray to provide competitive action for British players while the professional tours remain suspended because of the coronavirus pandemic.
After her defeat, Konta confirmed she is pulling out of next week's Palermo Open - the first WTA event since March - to focus on events leading up to next month's behind-closed-doors US Open.
The 29-year-old is the highest-ranked player competing in the Battle of the Brits, which sees the two teams - led by non-playing captains - chasing points in singles, doubles and mixed doubles.
Former world number one Andy Murray is only playing doubles because he does not want to take any risks with his fitness.
The 33-year-old Scot, who has dealt with pelvic and shin injuries since major hip surgery last year, is also continuing to prepare for the US Open in New York.
Monday's results/order of play
* Results/matches show Union Jacks player(s) first
Centre court:
Jodie Burrage 6-4 6-3 Johanna Konta
Dan Evans 6-4 3-6 7-10 Cameron Norrie
Anton Matusevich 5-7 3-6 Kyle Edmund
Katie Boulter v Emma Raducanu
Jamie Murray/Heather Watson v Joe Salisbury/Harriet Dart
Court one:
Ryan Peniston 4-6 6-3 7-10 Aidan McHugh
Lloyd Glasspool/Naomi Broady 6-4 6-2 Dom Inglot/Eden Silva
Heather Watson 7-5 6-4 Beth Grey
Dan Evans/Lloyd Glasspool v Dom Inglot/Alastair Grey