South Korean pair Chun In Gee and Park Sung Hyung set the pace at the final major of the year as both shot opening eight-under 63s to share the lead at the Evian Championship.
Chun, last year's US Women's Open champion whose best finish this season is joint second at the ANA Inspiration - the first major of 2016 in April - admitted to experiencing some anxiety before she teed off.
It did not show in the world No.7 play, however, as she hit every fairway in regulation and after a slow start off the 10th tee she carded six birdies on her second nine to record eight in all in her bogey-free round.
"Before the round I felt really nervous today - I think it's because it's the last major - and it was windy and there was a thunderstorm this morning so I tried to focus more on my game," said the 22-year-old, whose score was two short of the course record.
Late in the day Park carded six birdies over her final 10 holes to joint her compatriot at the top of the leaderboard with a one-stroke advantage over Annie Park.
The American, one of the afternoon starters as the weather improved at Evian Resort Golf Club, had seven birdies as she also returned a blemish-free card.
World number one Lydia Ko could manage only a 70 for one-under, seven off the lead.
Sarah Jane Smith is the highest-placed Australian, tied for 15th spot with a cluster of players having carded a two-under-par 69.
Lydia Ko is a shot behind in 28th spot, with Minjee Lee in 39th after a level-par round of 71.
Su Oh is two-over-par.
