Pennsylvania - Brittany Lang battled through the unrelenting heat to fire a two-under 69 and take a one-shot lead over South African Kelli Shean after the first round of the US Women's Open.
Lang, who is seeking her first win on the LPGA Tour, bogeyed her final hole on Thursday but still finished just ahead of a group of four golfers including Shean.
Shean was alone in front for over half the round but she three-putted on her final hole at the Oakmont Country club to drop to minus-one.
South Korean Park Inbee is in second with Shean and three others one shot adrift of Lang.
"You said it couldn't be done, but on any golf course you can shoot a low number," Lang said. "If you're hitting the ball good, you can for sure shoot a low score out here. But if you're not on your game you can shoot a big one."
Lang's round included rolling in a 40-foot putt on the 442-yard No. 15 to grab a two-shot lead.
Shean and South Koreans Park Inbee, M.J. Hur and Amy Yang are all at one-under 70.
Japan's Sakura Yokomine and Scotland's Mhairi McKay were alone at even-par 71.
Sweden's Sophie Gustafson and Canadian Alena Sharp were among a more-than-crowded group of 14 at one-over 72.
Lang has twice been a runner-up on the LPGA Tour and has more than two million dollars in earnings but no victories.
"But I know that if I think about winning the tournament, I probably will not win it," she said.
The 22-year-old Shean attends the University of Arkansas.
She developed her game after joining junior programme sponsored by South African compatriot Ernie Els.
"He taught me all the things I needed to know," Shean said. "Being able to interact with him and have any kind of relationship with Ernie Els is unbelievable," Shean said.
Shean's caddie, Chandler Rackley, is also her boyfriend.
Shean said having him nearby means she's "having the best time of my life out there."
Leading first-round scores on Thursday from the US Women's Open (South Korea unless noted):
69 - Brittany Lang (USA)
70 - Kelli Shean (RSA), Amy Yang, Inbee Park, M.J. Hur
71 - Mhairi McKay (GBR), Sakura Yokomine (JPN)
72 - Heekyung Seo, Paula Creamer (USA), Kristy McPherson (USA), Wendy Ward (USA), Shinobu Moromizato (JPN), Sophie Gustafson (SWE), Shi Hyun Ahn, Song-Hee Kim, Jeong Eun Lee, Alena Sharp (CAN), Cristie Kerr (USA), Christina Kim (USA), Jee Young Lee, Vicky Hurst (USA)
73 - Chella Choi, Tamie Durdin (AUS), Sandra Gal (GER), Maria Hjorth (SWE), Jeong Jang, Sarah Kemp (AUS), Seon Hwa Lee, Lisa McCloskey (COL), Na On Min, Ai Miyazato (JPN), Suzann Pettersen (NOR), Jennifer Song, Louise Stahle (SWE), Yani Tseng (TPE)
74 - Chie Arimura (JPN), Danielle Kang, In-Kyung Kim, So Yeon Ryu, Karrie Webb (AUS)
75 - Na Yeon Choi, Julieta Granada (PAR), Katherine Hull (AUS), Azahara Munoz (ESP), Karen Stupples (GBR), Alison Walshe (IRE)
76 - Feng Shanshan (CHN), Maria Hernandez (ESP), Candie Kung (TPE), Jiyai Shin, Victoria Tanco (ARG)
77 - Tania Elosegui Mayor (ESP), Eun-Hee Ji, Sarah Lee (GBR), Mika Miyazato (JPN), Becky Morgan (GBR), Anna Nordqvist (SWE), Se Ri Pak, Anna Rawson (AUS), Christine Wong (CAN)
78 - Janice Moodie (GBR), Belen Mozo (ESP), Hee Young Park, Jennifer Rosales (PHI), Lindsey Wright (GBR), Sun Young Yoo, Nicole Zhang (CAN)
79 - Irene Cho, Karine Icher (FRA), Sara-Maude Juneau (CAN), Meena Lee, Teresa Lu (TPE), Gwladys Nocera (FRA), Juliana Murcia Ortiz (COL), Pornanong Phatlum (THA)
80 - Hye Jung Choi, Veronica Felibert (VEN), Catriona Matthew (GBR), Sun Gyoung Park, Giulia Sergas (ITA), Marianne Skarpnord (NOR), Momoko Ueda (JPN)
81 - Helen Alfredsson (SWE), Sofie Andersson (SWE), Kirby Dreher (CAN), Junthima Gulyanamitta (THA), Ariya Jutanugarn (THA), Joo Mi Kim, Jenny Shin, Eunjung Yi
82 - Esther Choe, Ji Young Oh
83 - Kyeong Bae, Feng Yueer (CHN), Louise Friberg (SWE), Hee-Won Han, Sakurako Mori (JPN), Aiko Ueno (JPN)
84 - Lorie Kane (CAN), Bo-mee Lee, Nicole Vandermade (CAN)
86 - Mi Jeong Jeon