Boulter does well to save the break point and move ahead to take the hold.

A break of serve would not be ideal at this point in the match.

Melanie South
Former British Fed Cup player on BBC TV

Pegula has came out here with real purpose. She's not allowing Boulter to lead the pace of this match.

Pegula earns a break point in Boulter's opening service game but the Brit saves it with an overhead smash and lets out a big roar.

The Brit is trying to get herself going in this match.

Pegula gets her first game on the board with a hold to 15.

Solid ball-striking from the American.

Off we go then.

A must-win match for Great Britain and Boulter. Victory for the US would give them an unassailable 2-0 lead in the tie.

* denotes next server

It has been a difficult season on the tour for British number two Katie Boulter, and an opening-round loss at the US Open saw her drop out of the top 50.

But she has always been impressive when representing her country, winning 17 of 23 matches played for Britain.

Her latest appearance against Japan's Moyuka Uchijima sealed GB's place in the semi-finals.

But up against American Jessica Pegula, she faces one of the most consistent players on the tour.

The world number seven has not dropped out of the top 10 for three years, reaching the semi-finals of the US Open last month.

Pegula lost her singles rubber to former Wimbledon champion Elena Rybakina in the quarter-final against Kazakhstan.

However, the 31-year-old recovered to win the deciding doubles match alongside Taylor Townsend as the US pursue a record-extending 19th title.

Slightly earlier than I suggested, apologies. Boulter and Pegula are out to warm up for match two of this semi-final tie.

We'll be back in around 15 minutes for the second singles match where British number two Katie Boulter takes on world number seven Jessica Pegula.

It's a must-win match if Great Britain are to stay in this semi-final tie against the United States.

Emma Navarro speaking after her win over Sonny Kartal: "I had a ton of support from the ladies in my team. I feel a ton of pride playing for my country, it means the world to me to have the American flag on my chest.

"I was hearing you guys cheering me on. Great Britain was pretty loud in this match, I needed all the help I could get. Thanks for cheering me on."

Russell Fuller
BBC tennis correspondent

That was Sonay Kartal’s first-ever experience of defeat in a Billie Jean King Cup match, after three wins out of three in this debut season.

She was very good, aggressive throughout and only faded slightly in the final few games of the match.

Emma Navarro has had a stressful week, but after saving match points to beat Yulia Putintseva in the quarter-finals, showed again what an asset she is to this American team.

A brilliant, resilient performance from Kartal. She will be gutted to have lost that after taking the first set. Her first BKJ Cup defeat.

Navarro steps up to the microphone for her post-match interview wearing a USA headband.

Peter Odgers
BBC Sport commentator

It's a gutsy, gritty, resilient win for Navarro. USA are one win away from the final. Fantastic effort from Kartal, she played well above her world ranking.

Oh my. A double fault while serving for the match wipes out the first match point.

Nerves?

Kartal takes the first point on the Navarro serve after an incredible rally but the American recovers and earns two match points after Kartal's volley drops long.

Over to Navarro to serve for the match.

But the way this match has gone so far, anything could happen.

Kartal has come out swinging on serve and a superb shot that kisses the line moves her to 40-0 before a brilliant backhand seals the hold to love.

That will give the Brit a boost.

There have been a lot of comebacks across the Billie Jean King Cup this week and Kartal would have to produce a pretty special one to prevent Navarro taking this one.

A first point on the board for two games is a good start.

More tired shots from Kartal and it feels like the match is just running away from the Brit.

Navarro wins an eighth point in a row to take the hold to love and puts herself one game away from putting a point for the United States on the board.

Another error from Kartal and it's a break to love for Navarro.

So much fight on both sides of the net but it feels like Kartal is just starting to waver physically.

Navarro has a two-game cushion again.