Lewis and Park share Evian lead
Lewis, the 27-year-old American, birdied the 18th from 20 feet to rescue a one over par score on a day of heavy showers. Park also made four at the long final hole for a 70 and the pair were tied on 11 under par.
Three players shared third place on ten under - Australia's Karrie Webb, the 2006 champion, shot 67, American Natalie Gulbis, the 2007 winner, fired a 68 and the 17-year-old South Korean amateur, Kim Hyo Joo, had a fine 69.
Lewis, seeking a third win of the season, was three ahead after birdies at the ninth and 12th. But she bogeyed the 12th and 14th - just as she had done on Friday - and then took a double-bogey seven at the long 15th.
With a duck and her ducklings wandering on the fairway, a distracted Lewis chunked her third shot short of the green. She then mishit the next chip and ran up the ugly seven.
But she was delighted to hit back with the birdie at the 18th to stay at the top.
"I was so pleased to make the birdie at the last and make sure I was in the last group tomorrow," she said.
Park, the 2008 US Women's Open Champion, also had a hiccup with bogeys at the 11th and 154th. But she had four birdies - and came close to an eagle at the 18th where she hit her second shot to 12 feet.
"Because of the rain and the weather, today was by far the toughest day," said Park. "It is nice to see a Korean amateur (Kim) up there. I have never played with her but her game must be very good."
Webb is the former world No.1 with 38 LPGA titles in a stunning career and she attributed the better score to form with her putter.
"Today was my best round but I actually didn't hit the ball so well," admitted the 37-year-old seven-time major winner whose last win was in the LPGA Founders Cup in March last year.
"But I holed a couple of nice long ones. I've actually been playing well all season and it would be great to win here again."
Gulbis's triumph five years ago is her only victory on Tour in a career which has been blighted by injuries.
"I always love coming back here," she said. "It always brings so many good memories flooding back. It would be perfect to win again tomorrow."
Kim had five birdies and a bogey on the front nine, but fell back with bogeys at the tenth and 13th before sneaking back into firm contention with birdies at the last two holes.
Evian has a good record for amateurs. In 2005, Michelle Wie finished second and in 2010 Lexi Thompson was the joint runner-up.
France's Karine Icher produced her best round of the week, and a five birdie 68 saw her move into a tie for 19th on six under par.