Jason Day has iced his season of resurgence in the United States with a victory alongside Lydia Ko in the Grant Thornton Invitational pairs event.

They shot 26-under par over three rounds to win the alternative format event by a shot from Canadians Brooke Henderson and Corey Connors at 25-under.

They will share the $US1 million first prize in the inaugural event sanctioned by the PGA TOUR and the LPGA Tour.

Day and Ko began the final round of four-ball at Tiburon Golf Club in Naples, Florida with the lead but they were joined at the top by Henderson and Connors late in the day.

Ko’s stunning three wood second shot close to the flag at the par-5 17th, setting up birdie, was the moment when the ANZAC duo reached the outright lead again, and they only needed Ko’s steady par at the last to complete the win.

“It was weird, because it felt like the most stress-free win because I knew that she (Ko) was going to step up in the end, which was fantastic” said Day afterward. “It was a fun week, having the two tours join this week was a fun way to finish the year and I’m hoping we can do it for a very long time.”

Day is now the top-ranked Australian in the male world rankings at No. 19, and potentially in the slot to earn a berth at the Paris Olympic Games.

His season was highlighted by a win in the Byron Nelson tournament in May.