Cameron Norrie beat Rafael Nadal for the first time and Katie Swan beat Nuria Parrizas-Diaz to put Great Britain ahead 2-0 against Spain in their United Cup group stage encounter.
Norrie came from a set down to beat the 22-time Grand Slam champion 3-6 6-3 6-4 at Ken Rosewall Arena in Sydney.
The 27-year-old had lost all four previous encounters with Nadal.
“He’s bet me easily the last few times, so that’s a great way to end the year for me,” Norrie said.
“I thought I had never won a set before against him and I came out first wanting to do it.
“I had to stay super patient. It was a super physical match and I enjoyed it. It was huge to pass that one and finally beat him. It was a crazy match.”
Australian Open champion Nadal, 36, came through the first set, but Norrie broke Nadal’s serve in game six of the second set and held on to level the contest.
World number 14 Norrie broke early in the deciding set and saved two break points while leading 3-2 before serving for the win.
Nadal will look to defend his Australian Open title when the first Grand Slam of the year kicks off in Melbourne on January 16.
“All credit goes to him for doing a lot of things very well,” Nadal said.
“There is room for improvement but I have time before the start of the Australian Open in two weeks.”
Swan, 23, also came out of a set down as she beat Parrizas-Diaz 3-6 6-1 6-2.
It was a confident performance from world number 145 Swan, who is ranked 75 places below 31-year-old Parrizas-Diaz.
After a 3-0 win over Australia in their opening group stage, Britain need a win from their three remaining matches against Spain to reach the knockout stages.
World number 13 Paula Badosa takes on Harriet Dart in the opener on Sunday before Dan Evans takes on Pablo Carreno Busta, with the deciding mixed doubles to follow if needed.
The first mixed team tournament features six round robin groups of three teams.
Great Britain, Spain and Australia are vying for a chance to face the United States, Germany or the Czech Republic in a play-off to reach the semi-finals.
