Rochus advanced with a 6-3, 6-2 victory over Argentina's Brian Dabul, while Fish defeated Britain's Richard Bloomfield 7-6 (7/5), 6-4.
Fish booked his second finals berth of the season, after falling to compatriot Sam Querrey in the final at Queen's Club last month.
Rochus, who will be playing his eighth career final, said he was expecting a tough match.
"He's a great player. He went to the final at Queen's," Rochus said of Fish. "He's a good player on grass. I'm here in the final. I have nothing to lose against Mardy. It's going to be a very difficult match."
Bloomfield, ranked 552nd in the world, dropped his serve in the ninth game, the first service break of the match coming as he delivered faults on eight of his nine serves.
"I'd like to think I put a little pressure to be able to do what he did in that game at 4-all," Fish said. "You have to hold serve out here. If you don't hold serve, you're not going to last."
Fish closed out the match with his fourth love service game of the set.
"I continued to concentrate as much as I could on my service games," he said. "I think I was aggressive on my service games."
Neither player had been broken in the first set, but Fish gained the upper hand in the tiebreaker with two aces and closed it out wit a forehand cross-court winner.
Bloomfield, was playing in his first ATP semi-final. He was 1-10 in his career before his three victories here this week.
"It has been a great week, a fun week. I'll remember it forever," he said.
But the Briton's first-round victory over Christophe Rochus was being scrutinized by tennis officials after online gambling exchange Betfair reported that the match attracted unusual action.
Bloomfield, 27, was also caught up in a betting controversy after his only ATP win before Newport - a 6-1, 6-2, 6-2 victory over Argentina's Carlos Berlocq in the first round at Wimbledon in 2006 that was investigated following irregular patterns.
"I said to him before the match, the only two matches he's won in his career and there's been a gambling scandal," Fish said. "He doesn't seem like that type of guy. Maybe he's got rich friends."
Fish and Bloomfield played after an on-court ceremony in which Australia's Todd Woodbridge and Mark Woodforde - the "Woodies" - headed a class of seven inducted into the International Tennis Hall of Fame.