Sabalenka won 6-3, 3-6, 6-4 Saturday after CoCo Vandeweghe had beaten Aliaksandra Sasnovich 6-4, 6-4 to give 17-time champion United States a 1-0 lead with the Americans looking to break a 17-year championship drought.
Sabalenka won on a fourth match point when Stephens netted a forehand. A wild second set saw six straight breaks of serve before Sabalenka held in the seventh game.
Stephens has yet to win any of the five matches she's played since winning the US Open.
Earlier, Vandeweghe faced a boisterous crowd at the indoor hard court in Minsk.
"I think that's part of having the home-court advantage, and I would expect the same in the U.S.," Vandeweghe said.
Vandeweghe broke in the opening games of both sets at Chizhovka Arena and didn't face a break point on her serve in the opener.
In the second set, Vandeweghe lost serve in the second and eighth games but broke serve on three of four offerings to secure the match.
Vandeweghe holds a perfect record in Fed Cup action this year, winning five singles and one doubles match, including her defeat of Sasnovich.
The Americans won a record 17th Fed Cup title in 2000, with Billie Jean King as captain. Since then, they have played and lost in three finals -- 2003, 2009 and 2010.
Belarus is playing in the Fed Cup final for the first time.