GB captain Anne Keothavong rushed over to help a struggling Johanna Konta to her feet at the end of the match
Johanna Konta battled through fatigue to send Great Britain into April's Fed Cup promotion play-offs, with her win over Aleksandra Krunic sealing victory over Serbia.
Konta, seen lying on the floor in the gangway before the start of the third set, needed to be helped to her feet at the end of her 7-6 (7-1) 3-6 6-2 win.
Her team-mate Katie Boulter had earlier dismissed Ivana Jorovic 6-4 6-3.
GB are one win away from reaching World Group II for the first time since 1993.
Victory completed a perfect four days for GB in their first Fed Cup matches on home soil for 26 year years, as they won all four of their ties in Bath without losing a single rubber.
A determined Konta showed terrific spirit to find enough energy to win her third three-set battle in as many days, having been on court until after 23:00 GMT on Friday.
GB captain Anne Keothavong told BT Sport: "The guts, the courage by Jo and Katie today... Physically, if I could tell you what's going on behind the scenes.
"These girls are so strong. I don't know what to say. I'm just so proud."
Konta steadies herself to claim victoryJohanna Konta's mum Gabriella rushed down to the court to give her daughter a hug
Following a relatively slow start, Konta fought back from a break down early on and then raced in to a 6-0 lead in the first-set tie-break before Krunic went long with a backhand on the Briton's second set point.
But a determined Krunic looked fresher and sealed the second set when Konta netted a backhand after a strong defensive recovery from the Serb at set point.
However, after a comfort break - during which an exhausted Konta was seen on the floor - the British number one steadied herself remarkably to move a break up to 3-1 in the third set.
And she broke again in the final game to take the set 6-2, emotionally sinking to the floor after sealing it with her second match point.
Her watching parents were visibly upset and her mum Gabriella rushed down to the court to give her daughter a hug.
"I'm feeling pretty rubbish but it's OK," Konta said.
More to follow.
