ST ANDREWS, August 1, 2013 (AFP) - South Korea's Park, the world number one, is burdened with the pressure of trying to become the first player to win four majors in one calendar year and a shaky patch tarnished a seven birdie round over the Old Course at St Andrews.
Out in five under par and with a day's sixth birdie at the tenth, the 25-year-old from Seoul was in her usual position at the top of the leaderboard.
But four dropped shots in five holes from the 13th left her feeling a little frustrated.
"I am a little disappointed," she admitted after making a seven foot birdie putt at the 18th.
"I had a couple of bad drives and a couple of bad putts.
"I was nervous at the start because I know what I am playing for. I made a few birdies and that gave me confidence but I had a shaky spell and I definitely hope to improve over the next three days."
The 16th was the real blemish.
She dumped her second shot into an awkward lie in a bunker, hit out away from the flag and then three-putted for a double-bogey six.
Pressel has her own goal. She needs a decent week to force her way into the US Solheim Cup for the match against Europe in Colorado in two weeks' time.
Pressel is currently number 51 in the world and needs to overtake Jennifer Johnson -- the number 50 who shot 74 on Thursday -- to automatically make the line-up for a third successive time.
The team is named on Sunday.
"If I had a dollar for every time I think about the Solheim I would be a zillionaire," she joked.
"It's such a special week and I don't want to miss it.
"I'm trying not to dwell on it. But it has been nice this week to get some encouraging notes and messages from other players."
Lennarth, a Monday qualifier, was the surprise package.
Late in the day, the rookie on the Ladies' European Tour birdied the 18th from ten feet to join Pressel at the top.
"This is my first British Open and it has always been a dream to play here at St Andrews," said the 25-year-old.
"I am surprised to be joint leader but I know I can hit good shots and play good golf.
"I birdied the first and that helped me relax and now I'm looking forward to tomorrow."
In a cramped leaderboard, Pressel and Lennarth led by one shot lead from a group of seven players that included two former US Open champions from South Korea, Ji Eun-Hee and Choi Na Yeon.
Scotland's Catriona Matthew, the 2009 champion, played with Pressel and kept up hopes of a home win with a fine 68 that included birdies at the final two holes.
The third member of the group was defending champion Shin Jiyai and the South Korean opened with a 71.
Leading first round scores of the women's British Open at Saint Andrews on Thursday (par 72, GBR & IRL unless stated):.
66 - Morgan Pressel (USA), Camilla Lennarth (SWE)
67 - Stacy Lewis (USA), Choi Na Yeon (KOR), Jeon Mi-Jeong (KOR), Nicole Castrale (USA), Ryann O'Toole (USA), Sydnee Michaels (USA), Ji Euen-Hee (KOR)
68 - Paula Creamer (USA), Elizabeth Young, Danielle Kang (USA), Georgia Hall *, Catriona Matthew, Lizette Salas (USA), Pernilla Lindberg (SWE), Dori Carter (USA)
69 - Park Inbee (KOR), Sandra Gal (GER), Ai Miyazato (JPN), Lydia Ko (NZL)*, Feng Shanshan (CHN), Caroline Hedwall (SWE), Katie Burnett (USA), Katherine Hull-Kirk (AUS), Florentyna Parker, Ryu So Yeon (KOR), Ayako Uehara (JPN), Miki Saiki (JPN), Angela Stanford (USA), Shin Jenny (KOR), Seo Hee-Kyung (KOR), Mikaela Parmlid (SWE), Marianne Skarpnord (SWE), Mariajo Uribe (COL), Malene Jorgensen (DEN), Oh Ji Young (KOR)
* - denotes amateur