Ainslie, the six-time Finn world champion, is sitting second overall but the star of the day was Denmark’s Jonas Hogh-Christensen, who won both races.
A notoriously slow starter in his previous Olympic campaigns, Ainslie struggled upwind but made huge advances on every downwind leg when his strength came to the fore.
But Hogh-Christensen, himself a double Finn world champion, looked untroubled throughout as he bids to stop Ainslie taking another gold that would see the Brit become the most successful Olympic sailor ever.
That title currently belongs to another Dane, Paul Elvstrom, who won four gold medals between 1948 and 1960 and Hogh-Christensen has stated his desire to defend his countryman’s record.
But Ainslie was delighted with his start.
“It was a good start, but there’s a long way to go and I have to keep plugging away over the coming days,” he said.
“Jonas sailed really well - he really nailed both those first beats and got away – he’s obviously one of the guys who can do that on his day and we’ll see how things develop throughout the week.
“He was going pretty well and hats off to him – and once he got away he had a bit of a cushion.”