Isner put the U.S. ahead with a 6-4, 6-1 win over Christian Lindell, who was playing only his second ATP tournament. Robin Soderling rallied to tie it up with a 5-7, 7-5, 7-6 (3) win over Sam Querrey before Isner and Fish clinched it with a 4-6, 7-6 (5), 10-4 victory over Simon Aspelin and Robert Lindstedt.
Under gray skies, Isner broke serve for 2-1 against Lindell but then had to fight off a break point in the next game before taking the first set. The American broke again for a 2-1 lead in the second set, held off two break points to go ahead 3-1 and then easily closed out the match.
"It's pretty slow but it makes you hit a lot of balls so it's good," said Isner of the heavy clay surface. "The weather's a little cold right now but hopefully it'll warm up.
"I was eager to get out on the court and just play my game. I love it here. It's awesome, perfect prep for the French [Open]."
Soderling, who has a 9-1 record in Duesseldorf, was trailing 5-4 in the second set before he rallied. He won the next three games to level the match. He came up with three consecutive breaks and led 4-2, but Querrey forced a tiebreak before Soderling finally prevailed.
Also, Germany beat Serbia 3-0.