Emma Raducanu got Great Britain off to a winning start but defeat for Katie Boulter means their Billie Jean King Cup semi-final with Slovakia goes to a doubles decider.
Raducanu, who also triumphed in straight sets in Britain's prior wins over Germany and defending-champions Canada, defeated Viktoria Hruncakova 6-4 6-4,
But, despite taking the opening set against world No 43 Rebecca Sramkova, British No 1 Boulter went down 2-6 6-4 6-4 in the second singles rubber.
Doubles pairing Olivia Nicholls and Heather Watson have not yet been required during the Finals, with Raducanu and Boulter having won both of their previous singles matches. They now face Hruncakova and Tereza Mihalikova, the winners securing a spot in the final against Italy.
Great Britain are hoping to reach the final of the competition formerly known as the Fed Cup for the fifth time, but for the first time in 43 years.
Raducanu gets GB off to perfect start
In the opening match of the semi-final tie, Raducanu came flying out of the blocks against Hruncakova, immediately breaking the world No 238.
Raducanu had to then fend off three break points on her own opening service game before opening up a 5-1 lead with a second break of her opponent.
The 2021 US Open champion faltered a touch when closing out the first set, spurning a set point when broken back, but eventually converted her third.
The second set followed a similar pattern, Raducanu earning a double break at 5-2, only to be broken back when serving for the match. But the 22-year-old eventually closed things out for a 6-4 6-4 win.
"Every match is extremely challenging, and I think as the tournament progresses it's even more so," Raducanu said in her on-court interview.
"Today was a really tough battle because my opponent has a huge ball strike and, despite her ranking, she plays much above that.
"I'm really happy to come through that because I've lost to her before [in 2023 in Auckland]."
Boulter beaten to force doubles decider
Boulter failed to back up Raducanu's third straight win with one of her own despite taking the opening set comfortably against her opponent who is ranked 19 places below her.
But Sramkova, who is almost blind in one eye, beat Boulter in their only previous encounter on clay in the spring and upset world No 11 Danielle Collins in Slovakia's defeat of USA in the first round.
As Boulter's groundstrokes became more erratic, Sramkova opened up a 4-1 lead in the second set and though the Brit steadied herself to win the next three games, she was broken again at 4-5 to force a deciding set.
The momentum was now with the Slovakian, who took an early lead in the third, only for Boulter to again peg her back.
The 28-year-old's resistance was finally ended, though, when again broken to stay in the set, Sramkova claiming the second of two match points when Boulter missed a backhand.
Italy await in BJK Cup final
Four-time champions Italy booked a second-successive appearance in the final with a 2-1 victory over Poland on Monday.
World No 4 Jasmine Paolini and her team-mate Sara Errani clinched the deciding doubles 7-5 7-5 against Iga Swiatek and Katarzyna Kawa after an extraordinary second-set comeback in Malaga.
Five-time Grand Slam champion Swiatek had battled back from a set down to defeat Paolini in their singles tie, after world No 78 Lucia Bronzetti struck first for Italy, defeating Poland's Magda Linette - ranked 40 places higher - in straight sets.
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