Great Britain's hopes of reaching the United Cup semi-finals rely on Katie Boulter beating five-time major champion Iga Swiatek after Billy Harris narrowly lost to former Wimbledon semi-finalist Hubert Hurkacz.
Harris, 29, pushed 16th-ranked Hurkacz in the opening match of the best-of-three tie before going down 7-6 (7-3) 7-5 in Sydney.
The late-blooming Harris, who is ranked 125th in the world after making huge strides in the past two years, was aiming to give Britain a platform to reach the last four with the biggest win of his career.
His defeat means 28-year-old Boulter must beat Swiatek later on Thursday to tee up a deciding mixed doubles match.