Stephens defeated Anabel Medina Garrigues 6-3 6-1 before Isner beat journeyman Daniel Munoz-De La Nava 6-3 6-4 in 67 minutes to give the second-seeded US an unassailable 2-0 lead over Spain.
Munoz-De La Nava said he had no answers to 208cm tall Isner's powerful serve, which regularly clocked above 200km/h.
"It's pretty tough because he's serving like an animal," Munoz-De La Nava said.
"He's changing the serve all the time. All directions.
"It's tough to return. I tried my best.
"But if you lose your serve against John, it's impossible to win."
Medina Garrigues struggled with sore knees on the way to a 6-1 6-0 loss to Petra Kvitova on Sunday.
But the 31-year-old felt much better after receiving anti-inflammatory injections on Sunday and before Monday's match.
However, Medina Garrigues was still no match for Stephens' power game.
After Medina Garrigues tied up the first set at 3-3, Stephens lost just one more game for the match as she raced to victory in 76 minutes.
"I felt good," Stephens said after her win in just 76 minutes.
"I thought it was a good way to start the year off.
"For years and years I've wanted to play at the Hopman Cup.
"I love it here and hopefully I'll get invited back."