Meghann Shaughnessy overcame teenager Kirsten Flipkens 6-7, 7-6, 9-7 to give the United States a commanding 2-0 lead over Belgium after day one of their Fed Cup semi-final on Wednesday.
Lisa Raymond had trounced Els Callens 6-2, 6-1 in the opening rubber.
Flipkens, playing in her first Fed Cup match, was the more consistent player early on as she overcame a nervous start and a hip injury to take the first set.
She was just two points away from victory, serving for the match at 5-4 in the deciding set. But Shaughnessy battled back to break her 17-year-old opponent in the 16th game to clinch the win on her first match point after three hours and 12 minutes.
Doubles specialist Raymond dominated Callens throughout their contest, clinching the win in just 50 minutes.
"I don't think I could have asked more of myself or I could have played any better," said the 30-year-old American.
"It's been a great week so far and I think we came here to win this cup."
The Americans have captured the Fed Cup a record 17 times.
Belgium are missing their top two players, world numbers one and two Justine Henin-Hardenne and Kim Clijsters, and are considered underdogs in the four-team tournament.
"It was a tough match and she played really well," said a dejected Callens, ranked number 74 in the world.
"I just hope to give a better performance in tomorrow's match against Shaughnessy."
Hosts Russia take on France in the second semi-final at Moscow's Olympiiski sports complex later on Wednesday.