Kupcho, ranked 53^rd in the world, notched nine birdies and bogeys on the eighth, 15^th and 16^th holes for a score of 66 from the first round at Mission Hills Country Club.

That was matched by Lee after her bogey-free round.

It is unchartered territory for the duo as neither have previously held a first-day lead at a major.

Only Sweden's Annika Sörenstam, now President of the International Golf Federation, has previously defended her title at the Chevron Championship and that was back in 2002, but Tavatanakit enjoyed a strong start back at Mission Hills.

Seven birdies, including two in the final three holes, featured in her round of 67 which put her five-under-par.

A group of six players are one shot behind on four-under.

Three-time major winner Anna Nordqvist of Sweden, Germany's Caroline Masson, Thailand's Pajaree Anannarukarn, Australia's Gabriela Ruffels, world number three Lydia Ko of New Zealand - the 2016 Chevron Championship winner - and England's Georgia Hall all scored rounds of 66.

World number one Ko Jin-young of South Korea endured a difficult day, and is tied for 78^th after two bogeys and no birdies on a round of 74.

Another former winner in Ko's compatriot Park Inbee also struggled with five bogeys, and finished three-over-par.

The Chevron Championship is golf's first major of the year, although this is set to be the last time it is staged at Mission Hills Country Club, with a move to Houston expected next year.

The second round is set to be held tomorrow.

Lee is due to play with her compatriot Hannah Green, and Kupcho with fellow American Lizette Salas.