Three more races are scheduled for Saturday.
Frost, 17, in his second season in the championship, thus notched his maiden win by topping Race-2 after having finished third in Race-1 that Defourny, 20, won from pole position.
In Race-1, Frost made five places to come in third and in the next outing, climbed from fourth on the reverse grid to win.
In contrast, Defourny dominated Race-1 with a lights-to-flag victory and in the next race, sliced his way through from sixth on the reverse grid to finish third behind pole-sitter Czech Republic's Petr Ptacek who did well to hold off the Belgian over the last couple of laps.
"I had a good start and made positions in the very first lap before taking the lead. Thereafter, I consolidated my track position and it was quite comfortable in the end," said Frost.
Earlier, Defourny put his pole position start to best use in Race-1 and was never seriously challenged even as he nursed his early lead though eschewing risk.
Behind him, Jamie Chadwick, 20, who started second, slipped a bit, losing a couple of track positions, but recovered to regain her second spot while Frost drove well to finish third from eighth on the grid.
India's Chetan Korada, making his debut in the championship, was placed 10th and 12th in the two races. He started Race-1 from the pitlane following an engine change, but said he was pleased to improve his lap times while getting used to the car.
Earlier, the qualifying session in the morning gradually developed into a two-way battle between Defourny and Chadwick who was provisionally on pole with just a few minutes left on the clock with a 01:57.177.
The 20-year old Belgian's response was a 01:56.819 which eventually ensured him the pole position. Chadwick, also 20, then replied with her best lap of the session, 01:56.937, but had to be content with the second spot on the grid.
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