Ben Ainslie won both races at a cold and windy Falmouth Bay to take a 10-point lead over fellow Britons Ed Wright and Andrew Mills at the halfway stage of the JP Morgan Asset Management Finn Gold Cup.
The 35-year-old Olympic gold medal hopeful said: "It was a great day and really awesome sailing but hard work. Tactically it was hard and physically it was harder but I had a good day."
The conditions are expected to be less testing on Wednesday, with very light winds forecast.