* 'Today my back really bad', says Jang
* World number one Park opens with a 70 (Adds details, quotes)
July 16 (Reuters) - Jang Ha-na of South Korea overcame a back injury to make five birdies on the last nine capped by one at 18 for a five-under 66 and a one-shot lead after the opening round of the LPGA Marathon Classic in Sylvania, Illinois.
Tour rookie Jang played while dealing with an injury that forced her to finish her swing with one hand off the club and caused her to bypass hitting warm-up shots and consider withdrawing from the event.
"Today my back really bad," said Jang, who had hit two full clubs extra into the greens for much of the round.
"I'm thinking fairway, in the fairway and the greens, because really narrow the fairway and very small the greens. So just thinking ... fairway and the green."
Jang, ranked 22nd in the world and a six-times winner on the Korean LPGA Tour, avoided a five-way tie for the lead with her birdie at the last at the 6,571-yard Highland Meadows Golf Club.
Tied for second were Australian Sarah Kemp, American Nannette Hill, Lee-Anne Pace of South Africa and Hsu Wei-ling of Taiwan.
Kemp held the lead for much of the day and was tied for the top spot before taking a bogey on her last hole, the ninth.
One more stroke back were a dozen players on 68 including Koreans Kim Sei-young, Jenny Shin and Mirim Lee, Americans Angela Stanford and Brittany Lang, Canada's Alena Sharp, Caroline Masson of Germany and Dewi Claire Schreefel of the Netherlands.
Jang, 23, began solidly but quietly with a birdie at the third hole and pars on the other eight in her front nine.
The Seoul native jump-started her homeward half with a birdie on 10 and followed with three birdies in a row from the 12th. After a bogey at 15, she birdied the last for sole possession of the lead.
Jang has four top-10 finishes this season, including a tie for second at the season-opening Coates Golf Championship.
World number one Park In-bee of South Korea was four shots off the pace after a one-under 70.
Cheyenne Woods, niece of Tiger Woods, got off to a hot start with two birdies and an eagle in her first nine, but had four bogeys on the back to finish with 71 along with hometown favourite Stacy Lewis, Paula Creamer and defending champion Lydia Ko of New Zealand.