Seeking their second final in three years, Belgium went ahead with Goffin, the world number 12, coming from a set down to beat Millman, ranked 185, 6-7(4) 6-4 6-3 7-5.
World number 20 Kyrgios eventually wore down Darcis, ranked 77, in a three and a half hour battle, winning 6-3 3-6 6-7(5) 6-1 6-2.
Meeting for the first time, Goffin and Millman were evenly matched early on, with the Australian, a last-minute call-up replacing Thanasi Kokkinakis, breaking Goffin's serve to set up a first set tiebreak at 6-6 that he won with relative ease.
Goffin relied on some masterful forehands to win the second set and level the match. A gruelling 12-minute game proved pivotal for Goffin, allowing him to go on to take the match.
The Belgian proved superior despite an injury to his left knee during the U.S. Open earlier this month, which was Belgium's chief concern going into the clay tie.
Asked whether the match had taken its toll ahead of the Sunday singles, Goffin told reporters: "I'll try to give everything, I must give everything."
Kyrgios, who was fined for bad behaviour during his U.S. Open first-round defeat in August, withstood some punishing rallies, dropping few points on his serve despite some booing from the 15,000-strong Belgian crowd.
"It was incredibly tough... I was disappointed in New York... I knew I had to come here and prepare, just block it all out," he told reporters.
Belgium are bidding to appear in the final again after losing out to Great Britain in 2015, while Australia are looking for a first title since 2003. Australia have not beaten Belgium since 1991.
The weekend's winners will face the victor of France and Serbia who play in nearby Lille.
(Reporting by Robin Emmott; Editing by Christian Radnedge)
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