Postpublished at 20:30 Greenwich Mean Time
Boulter 6-1 0-0 Siegemund*
Katie Boulter is on a roll and she doesn't want to stop. The British number one is keeping warm, practicing her serves while Laura Siegemund heads off court for a bathroom break.
Great Britain beat Germany 2-0 to reach last eight at Billie Jean King Cup Finals
Katie Boulter seals victory with dominant 6-1 6-2 win over Laura Siegemund
Emma Raducanu gave GB lead by beating Jule Niemeier 6-4 6-4
GB will face defending champions Canada in quarter-finals on Sunday
Emily Salley, Marissa Thomas & Emma Smith
Boulter 6-1 0-0 Siegemund*
Katie Boulter is on a roll and she doesn't want to stop. The British number one is keeping warm, practicing her serves while Laura Siegemund heads off court for a bathroom break.
Boulter 6-1 0-0 Siegemund*
Lucie Ahl
Former British number one
Great accuracy from Katie Boulter.
She really is serving well and finding her forehand.
What a set from Boulter, six games in a row.
Boulter 6-1 0-0 Siegemund*
Great Britain are one set away from the quarter-finals!
Katie Boulter has absolutely steamrolled her way through that opening set.
*Boulter 5-1 Siegemund
Katie Boulter switches up play, going for a drop shot that doesn't have the best execution, but Laura Siegemund doesn't react fast enough to get there.
An ace from Boulter is followed by a miss, but she brings up the first set points with a piercing forehand winner.
*Boulter 5-1 Siegemund
And this time Katie Boulter gets it as Laura Siegemund, on her second serve, fires long.
The British number one has the chance to serve out the set now.
Boulter 4-1 Siegemund*
Another double fault by Laura Siegemund, another break point for Katie Boulter.
Boulter 4-1 Siegemund*
Katie Boulter throws her head back in frustration as a net cord takes her first break point wide.
She can't get the second one over the net and we go to deuce.
Boulter 4-1 Siegemund*
A double fault from Laura Siegemund pulls Katie Boulter level at 15-15 before the British number one unleashes a fiery cross-court backhand winner.
Her German opponent sends the next point into the net, gifting Boulter two more break points.
Boulter 4-1 Siegemund*
Not much for captain Anne Keothavong to say to Katie Boulter at the change of ends.
They'll both be pretty pleased with how this is going.
Boulter 4-1 Siegemund*
Lucie Ahl
Former British number one
Katie Boulter is playing incredibly well. She is full of confidence right now and she is able to put it on the court here.
Boulter 4-1 Siegemund*
After a nervy start, it's all looking rather comfortable for Katie Boulter now.
A series of big, booming winners helps her to a very solid love hold.
*Boulter 3-1 Siegemund
Laura Siegemund has not yet won a point on her second serve. It looks to be a really weak area of her game that Katie Boulter exploits here to go 15-30 up.
Then to 15-40, two more break points to Boulter - and Siegemund hits one long after a rally. The Brit is in control.
Boulter 2-1 Siegemund*
Boulter is motoring now, getting to 40-0 on serve in a flash.
The Brit is the world number 24, facing the player ranked 84th in the world - and that gap shows as Boulter holds to love with a fine ace to finish.
*Boulter 1-1 Siegemund
Katie Boulter has a chance to break straight back here, as Laura Siegemund double-faults. 0-30.
A poor second serve from the German is punished by Boulter, and she has three break back points...
And Boulter needs just one! We are back on serve.
Boulter 0-1 Siegemund*
Not a great start by Katie Boulter, who finds herself 0-30 down in her opening service game.
She responds well, forcing Laura Siegemund into planting a forehand into the net to make it 30-30 - but the German has a break point opportunity...
And Boulter, after a rally, plants one from the baseline into the net. Oof.
Warm-ups are done, and the final pieces of advice from the captains have been given.
Katie Boulter v Laura Siegemund is under way...
Russell Fuller
Tennis correspondent on BBC Radio 5 Live and BBC Sounds
Against Germany, Britain have two higher-ranked singles players so would be the favourites but there are a lot of teams in this section of the draw that can beat each other, in all honesty.
The winners of this would play the defending champions Canada in the quarter-finals on Sunday. It's a good half of the draw to be in though because sides like Poland and Italy that have top-five players in Iga Swiatek and Jasmine Paolini are in the other half of the draw.
The favourites in this half, the United States, were beaten by Slovakia late last night so it's definitely a land of opportunity.
Jonathan Jurejko
BBC Sport in Malaga
While the BJK Cup and Davis Cup are being played on the same site in Malaga's 'sports city' complex, they are split across two venues.
The BJK Cup is being held in a smaller, temporary structure, which holds 3,560 people and was the practice courts for last year's Davis Cup Finals.
Outside it still looks like a corporate marquee for champagne guzzlers. But inside it looks fit to host top-class tennis.
The Davis Cup, however, is being held in the 11,500-capacity Jose Maria Martin Carpena Arena.
Officials say the decision to split the events was based on the need for consistency in the court surface and, therefore, necessary to maintain the integrity of the competition.
Some onlookers have suggested that it is more down to estimated crowd numbers and each event not sharing all the same sponsors/branding.
Some nations have already claimed their place in the last eight of the Billie Jean King Cup Finals.
Hosts Spain, holders Canada, 2023 runners-up Italy and wildcards Czech Republic all qualified for the Finals automatically and received a bye in round one.
On Thursday, Slovakia surprised 18-time champions USA with a 2-1 comeback win, while Japan moved past Romania.
And earlier today, world number two Iga Swiatek helped Poland reach the quarter-finals with a 2-0 victory over Spain.
#bbctennis
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