"Mardy is feeling better, having won a couple of matches. He's in a better frame of mind this week in Miami," Courier said.
The Americans also named big-serving John Isner and the leading doubles team of Bob Bryan and Mike Bryan for the April 6-8 tie on clay at the Monte Carlo Country Club.
Fish had failed to win consecutive matches this year before beginning to play more confidently at the ongoing Sony Ericsson Open, reaching the quarter-finals after working with a new coach.
"There is a new voice in his ear. That always hits a bit of a refresh button for a player, which it looks like he needed after the start of the season," said Courier. "He's not playing his best tennis yet, but he's playing better and competing a lot better."
The U.S. team advanced to the Davis Cup quarter-final by going on the road to beat a Roger Federer-led Swiss team 5-0.
Courier said he felt no pangs of regret about passing over Davis Cup veteran Andy Roddick, who flashed some of his old form in upsetting Federer this week.
"Look, Andy said pretty clearly after we had our victory in Switzerland as a team that Mardy and John Isner deserved the right to continue on with what they started," said Courier, who added that Roddick could be considered for the team later in the year. "I agree with Andy's assessment."