Johanna Konta has collapsed on court but recovered to lead Britain to Fed Cup victory over Serbia, while France and Belarus are in command of their respective first round ties against Belgium and Germany.
Australia are a rubber apiece against the US thanks to a 6-1 7-6 over Sofia Kenin by Ashleigh Barty, while the Czech Republic and Romania have also split their opening singles matches.
Konta had a miraculous recovery after seeming down for the count midway through her decisive match with Serbia's Aleksandra Krunic in Bath.
Having spent almost 10 hours on court over the previous four days, the 27-year-old's capitulation seemed certain to push the tie to a deciding rubber.
She needed to steady herself on her chair before falling to her hands and knees on her way off court, where she was helped by volunteers and paramedics.
But the British number one returned seven minutes later after a medical assessment and produced a spirited comeback to earn a 7-6 (1) 3-6 6-2 win, which sealed a 2-0 success in the best-of-three tie.
"I started progressively feeling more and more unwell," Konta said.
"Light-headed, feeling shaky, feeling a little bit out of body. It got the better of me at the end of the second set. I tried not to panic and tried again to assess what I could and do the best I could.
"I was going to go as far as I could."
Katie Boulter earlier put Britain in the lead, beating Ivana Torovic 6-4 6-4.
Also on Saturday, Australian Open semifinalist Karolina Pliskova won the first point for the Czechs after easing past Mihaela Burzanescu 6-1 6-4 on an indoor hard-court in Ostrava.
But Simona Halep won the second singles by routing Katerina Siniakova, the star of last year's final against the United States, 6-4 6-0.
Pliskova and Halep, both former world No.1s, will meet in the key first reverse singles on Sunday.
In Liege, Caroline Garcia returned to the Fed Cup for the first time since the 2016 final to overcome Alison Van Uytvanck 7-6 (7-2) 4-6 6-2 and give France the first point against Belgium.
Alize Cornet made it 2-0 after defeating Elise Mertens 7-6 (7-6) 6-2.
Belarus, finalists in 2017, took control in Braunschweig after Aliaksandra Sasnovich beat Tatjana Maria 7-6 (7-3) 6-3 and Aryna Sabalenka dispatched Andrea Petkovic 6-2 6-1.
Belrarus are one win away from avenging losing to Germany 3-2 last year in Minsk.
In the last World Group tie, 2018 finalists the United States finished with a win and a loss against the Australians after Maddison Keys beat Kimberly Birrell 6-2 6-2.
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