Great Britain pulled off a huge upset to reach the Billie Jean King Cup semi-finals for the first time in 41 years.

Singles victories for Harriet Dart and Heather Watson against much higher ranked opponents had set up a tense finale with GB needing a clean sweep.

Great Britain will face Australia in the last four on Saturday.

Watched by their team-mates, who could barely sit still in their seats in the nervy moments and leapt out of them in the exciting ones, Barnett and Nicholls hugged each other before joining the others in a celebratory huddle.

"This is something really special," Keothavong said. "I'm just so proud of this team, every single player has done themselves proud, the team spirit has been so high."

Barnett added: "It's pure passion out there. These girls are playing for each other, we've had so much fun. There have been rippers out there."

Ranked lower than both of their Group C rivals, and without the injured Emma Raducanu, Great Britain had been described as underdogs by their captain Anne Keothavong and many others before this week's finals began.

When they lost their opener against Kazakhstan on Tuesday, they faced even more of an uphill challenge to advance and only a 3-0 win against five-time champions Spain would put them through.

But a fired-up Watson was in a ruthless mood, winning seven games in a row on her way to a 6-0 6-2 win over Nuria Parrizas Diaz, who is ranked 61 places above the Briton.

It was then over to world number 98 Dart, who stunned 13th-ranked Paula Badosa 6-3 6-4 to leave Great Britain tantalisingly close to an unlikely victory.

While Dart said her own performance had been inspired by Watson's and the home crowd, that same momentum carried on into the doubles where Nicholls and Barnett - who earlier in the week had simply spoken of their excitement of being involved at all - took centre stage and delivered in style.

They edged a close first set and then got an early break in the second as they sealed a 7-6 (7-5) 6-2 victory over Aliona Bolsova and Rebeka Masarova, Barnett hitting the winning volley on match point to trigger wild celebrations in the arena.

Great Britain surprise everyone - even themselves

There were tears all round after an emotional evening in Glasgow

Great Britain had not even qualified for the event initially, having been beaten by the Czech Republic in a play-off in April, and were only at the finals because they were later named as hosts.

Before that, they had endured years of playing in the lower tiers of the competition that was previously known at the Fed Cup and, with the team also losing 2021 US Open champion Emma Raducanu to injury around 10 days before the event, expectations were low.

Group rivals Kazakhstan after all were led by Wimbledon champion Elena Rybakina and Spain by one of the competition's highest ranked players in Badosa.

But with a roaring home crowd, team spirit and a bit of momentum, things started to look a bit different.

"I think a lot of people wrote us off," Nicholls said. "The camaraderie, we just wanted to do it for each other and I think it showed in the results today."

Great Britain have been runners-up in the event four times but have never won it.

Could there be another surprise in store?

"Through to the semi-finals, we have to allow ourselves to dream a little bit," Keothavong said.

Analysis

BBC tennis correspondent Russell Fuller

Britain played all day like a team with freedom, self belief and absolutely nothing to lose.

The required 3-0 victory over Spain seemed fairly implausible at the start of play.

But first Heather Watson, who hadn't won a match at full tour level since Wimbledon, beat a player ranked 61 places above her.

Then Harriet Dart beat the 13th best player in the world. Paula Badosa was ranked 85 places above her, but it was the world number 98 who was more reliable under pressure when the chips were down.

And finally, Alicia Barnett and Olivia Nicholls, who were barely inside the top 200 at the start of the year, made steering their country into a World Cup semi-final look easy.

GB have also landed in the right side of the draw.

In Ajla Tomljanovic, Australia will have the highest ranked player in the semi-final. But the United States, the Czech Republic, Canada and Switzerland - with their greater strength in depth - are all in the other half.

Australia await in semi-finals

Earlier in the day, Australia booked their place in the last four by beating Belgium 3-0 to win Group B.

Storm Sanders saw off Alison van Uytvanck 6-2 6-2 before Elise Mertens' exertions seemed to catch up with her.

Belgian Mertens, who won the WTA Finals in Texas on Monday night before taking three flights through six time zones to Glasgow, lost in three sets to Ajla Tomljanovic.

Mertens led Tomljanovic 6-4 4-1 with a double break in the second set but then lost eight games in a row before retiring at 3-0 down in the decider, seemingly hampered by shoulder and elbow issues.

Meanwhile, Canada dominated Italy to also win 3-0, with Leylah Fernandez spending a combined 90 minutes on court to win in singles and doubles, losing just two games.

The Canadians play Switzerland on Friday to decide who goes through as Group A winners.