Great Britain are one match away from securing promotion to the Fed Cup World Group after Johanna Konta beat Naomi Osaka 6-3 6-3 to give Great Britain a 2-1 lead over Japan.
Konta played superbly to overcome the in-form Osaka, breaking twice in the first set and once in the second.
Heather Watson will secure victory for Britain if she defeats Kurumi Nara in Sunday’s second singles in Miki.
The best-of-five tie will be decided by a doubles rubber if Watson loses.
If Britain win, they will be promoted to World Group II – the first time they would have a place among the top 16 teams in the competition since 1993.
“Naomi and I raised our level at different parts of the match so there was very little in it and really could have gone either way,” Konta said.
“I was happy I was able to stay tough when she was playing really well and raise my game when she dropped off a little bit.”
Anna Smith and debutant Gabi Taylor are due to play Miyu Kato and Makoto Ninomiya in the doubles but the line-up can be changed.
BBC tennis correspondent Russell Fuller
This was quite possibly Konta’s best performance since last year’s Wimbledon. She returned Osaka’s big serve impressively and hit a number of eye-catching backhand winners.
Stout defence was also crucial to frustrate Osaka and Konta produced some clutch serves on the rare occasions she found herself in trouble.
The British number one will almost certainly return later for the doubles if Heather Watson is unable to seal the tie for Britain before then.