Roddick was leading 6-3, 3-1 when Darcis stopped, sending the American into his first final since winning Memphis in February 2011. Roddick next faces third-seeded Andreas Seppi, who beat American Ryan Harrison 7-5, 6-1.
On the women's side, unseeded Tamira Paszek of Austria rallied to beat defending champion Marion Bartoli 4-6, 7-5, 6-4. Paszek's opponent in the final is fifth-seeded Angelique Kerber of Germany, who beat Klara Zakopalova of the Czech Republic 6-0, 6-3.
Players had great difficulty in gusts up to 50 mph.
Sixth-seeded Roddick was in control throughout against Darcis. The Belgian dropped his serve at the start of the second set after leading 40-0, and retired after playing three more games.
"I can't remember much tougher conditions that I've played in," Roddick said. "That wind out there is frightening. It's tough. It's not as much as about tennis today as just kind of getting through it.
"This week has worked out. Regardless of what happens, I'll go into Wimbledon with some wins behind me and some confidence on a surface I'm very comfortable on, you know, so I feel like I got my game back a little bit this week."
Harrison got off to a nice start, earning the first break to lead 2-1, but Seppi immediately broke back and broke again for the set.
After Seppi won the first two games of the second set, Harrison became increasingly frustrated and several times bounced his racket or hit the net.
Paszek will be playing in her first final since winning at Quebec City in 2010.
"It was very, very tough conditions," said Paszek, who was a quarterfinalist at Wimbledon last year but had won just two matches this season, losing 11 times in the first round.
"I just had to try to keep moving, keep my focus, and just try to stay in front of the line or like shortly behind the baseline."
Fourth-seeded Bartoli led 4-0 in the second set before Paszek dug deep to turn the match around.
After leveling the match at one set each, Paszek dropped her serve in the opening game of the final set. But she broke back in the next game.
After Paszek held serve to lead 3-2, Bartoli called for medical attention and had her left thigh heavily strapped. Paszek broke in the final game to reach her first grass-court final.
Bartoli said the conditions contributed to her injury, but she thinks she will be fine at Wimbledon next week.
"It was a nightmare to play in these conditions, honestly," she said. "Yeah, it (the leg) just got really pretty tight and then tired, but I think in two or three days it will just be back to normal.
"I felt like I got chances in pretty much every single game and ended up losing them."
In earlier quarterfinals postponed from Thursday, Roddick defeated Fabio Fognini of Italy 6-3, 3-6, 7-5, Seppi advanced when Philipp Kohlschreiber of Germany retired with a left ankle injury, Harrison beat Denis Istomin of Uzbekistan 7-6 (5), 6-4, and Darcis ended the run of Australian qualifier Marinko Matosevic 6-2, 7-6 (4).