An altercation between three drivers, Austrian Ferdinand Habsburg, Pukekohe's Taylor Cockerton and American Kory Enders on the first lap saw the race stopped.
From the restart, Leitch shadowed Armstrong while Daruvala was left to fight off the attack of Randle.
"I knew if I kept close to Marcus I could pressure him into a mistake," said Leitch.
Armstrong slid wide exiting a fast corner and Leitch was through. The Southlander then drove away.
"We've got to analyse what was wrong and fix it," said Armstrong who will again start from the front row today for the main race for the Denny Hulme Trophy.
Daruvala, who has pole for that race, said he decided not to chase the two Kiwis at the front yesterday.
"I was making too many mistakes and decided just to stay in front of Thomas."