Kyrgios delivered 20 aces during the 6-3, 3-6, 6-7 (5), 6-1, 6-2 victory over Darcis.
France and Serbia are also tied 1-1 in the other semifinal at Lille, France.
Kyrgios lost the second and third sets before rallying to deliver the much-needed victory in the three-hour, 36-minute match.
"It was incredibly tough," Kyrgios said afterward. "I knew it was going to be tough, especially on this very slow clay court. But I've done everything right.
"I made it my objective this week to get a win for Australia. I'm just glad I got my team a win today. Hopefully the boys can get it done in doubles tomorrow."
Belgium won the day's first match when David Goffin outlasted John Millman 6-7 (4), 6-4, 6-3, 7-6 (4) in a three-hour, 29-minute match.
"Physically I'm not at my best, but I knew for the team and the fans that I had to leave my heart out on the court," said Goffin, who has been hampered by a knee injury recently. "To give one point to your country is the best feeling you can have."
In the other semifinal, Serbia is without injured star Novak Djokovic (elbow) but received a stellar performance from Dusan Lajovic to surprisingly win the first match of the semi.
Lajovic rolled to a 6-1, 3-6, 7-6 (7), 7-6 (5) victory over Frenchman Lucas Pouille, who committed 70 unforced errors.
"The biggest advantage for me was to win the first set," Lajovic said. "It's always important in Davis Cup."
France evened things up later, when Jo-Wilfried Tsonga registered a 7-6 (2), 6-3, 6-3 win over Laslo Djere.
After fighting to win the opening set, Tsonga controlled the final two.
Both semifinals will feature doubles play on Saturday.
France's Pierre-Hugues Herbert and Nicolas Mahut will face Serbia's Filip Krajinovic and Nenad Aimonjic, while Australia's John Peers and Jordan Thompson meet Belgium's Arthur de Greef and Ruben Bemelmans.