updated 5:10 p.m. KOHLER, Wisconsin (AP) -Seventeen-year-old Lexi Thompson had a share of the clubhouse lead during the first round of the U.S. Women's Open on Thursday. Thompson shot a 2-under 70 and was tied with Beatriz Recari of Spain and Ai Miyazato of Japan among early finishers. Defending champion So Yeon Ryu finished with a 74 as players struggled in the nearly 100-degree (38 C) heat and high humidity at Blackwolf Run, a challenging 6,944-yard course in central Wisconsin. Cheyenne Woods, Tiger Woods' niece, had a 75. Top-ranked Yani Tseng has an afternoon tee time. Beyond the challenges posed by Blackwolf Run's layout, plus the heat and humidity, players also had to stay focused during six-hour rounds. "I didn't realize how slow it was going until I made the turn and it was three hours later," Thompson said. Thompson said she didn't feel like her group was playing slowly, and wasn't sure why rounds were taking so long to complete. "Maybe just the heat, and it was slowing a few people down," Thompson said. Several players said the heat affected their concentration. "You're not thinking 100 percent clearly all the time," said Paula Creamer, who shot a 1-over 73. Recari, who is from Spain and lives in Florida, said she was "soaking wet" but didn't really mind. "Doesn't bother me too much," Recari said. Just a matter of make sure that you hydrate yourself properly and drink plenty of water and take your electrolytes at the right time so you don't have any lulls out there. Meena Lee of South Korea, who finished 1 under, acknowledged that the heat was getting to her. "It was just too hot for me," Lee said, through a translator. Thompson, who became the youngest qualifier in U.S. Women's Open history at age 12 in 2007, is playing in her sixth Open. She has one career U.S. LPGA Tour victory, the Navistar Classic last year, but now she'd like to win a major. "It would mean a lot," Thompson said. That's a pretty big achievement right there. There's a lot of golf to be played. Three more days on a challenging golf course. So I'm just going to go out and try to play the golf course and try to do my best. Woods expected to get a call from her uncle after playing her first round in an Open. "He's always been supportive of my career and excited that I'm taking this next step," Woods said.