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.