Britain's Jamie Chadwick won the first of two W Series races in Austin, Texas, on Saturday to take a 10-point lead into Sunday's season finale of the all-female single-seater championship.
Winner of the inaugural title in 2019, the 23-year-old now has only compatriot Alice Powell standing between her and a second crown.
The pair, with three wins each, were previously level on points but Chadwick now has 134 to Powell's 124 after overtaking pole-sitter Abbi Pulling at the first corner and leading to the chequered flag.
Powell finished third, after starting 10th, behind Finland's Emma Kimilainen.
Chadwick will start Sunday's race from pole position at the Circuit of the Americas, with Powell ninth.
The series, which features on the support programme for the U.S. Formula One Grand Prix, aims to help female racers up the motorsport ladder. The champion will collect a US$500,000 cash prize.
(Reporting by Alan Baldwin in London; Editing by William Mallard)