* Schwartzel takes charge with a 10-birdie 65
* McIlroy faces uphill task to make cut
* Woods opens with a 71, Chinese teenager Guan with a 72 (Updates at end of round)
By Mark Lamport-Stokes
DUBLIN, Ohio, May 30 (Reuters) - While Rory McIlroy suffered a nightmare start, South African Charl Schwartzel regained welcome form with his putter to charge into a one-shot lead in Thursday's opening round at the Memorial Tournament.
Former Masters champion Schwartzel changed his putting grip after just four holes at a sun-drenched Muirfield Village Golf Club, the move reaping instant reward as he went on to fire a sizzling seven-under-par 65.
Long-hitting American Scott Piercy mixed eight birdies with two bogeys to card a 66 but world number two McIlroy, seeking his first victory of the year, battled to a 78 that included a four-putt for an ugly double-bogey at the par-three 12th.
Top-ranked Tiger Woods, a five-times winner of the elite PGA Tour event hosted by Jack Nicklaus, launched his title defence with a 71 on a muggy afternoon at Muirfield Village where the slick greens became tougher and bumpier for the late starters.
Schwartzel, who missed the cut at last week's Crowne Plaza Invitational, was delighted to regain form on the greens as he piled up 10 birdies, one bogey and a surprising double-bogey at the par-three eighth to take early control of the tournament.
"The birdie on the last definitely will make my evening a lot better," the 28-year-old South African told reporters after ending his round by sinking a curling 12-footer at the par-four ninth.
"I felt like I played really well, sort of ticking the ball over and got it to eight under," he said of his position with two holes remaining before he sliced his tee shot into the right greenside bunker at the eighth.
"Just had a bad one on No. 8, made a double. And that was the very worst I played all day. Today was one of those rounds where I really did flush it."
Schwartzel decided to change his putting grip after three-putting from just six feet to bogey the par-four 13th, and he promptly birdied the next four holes.
"I flushed it for the first four holes and two-putted for birdie on 11," he said. "And hit it close on 13, looked like I was going to go two under and I made bogey.
"But I went to the next hole, changed the grip and they (the putts) started pouring in. So it worked."
Schwartzel, the 2011 Masters champion, was one of only 12 players in the 120-strong field who dipped under 70 on a tricky day for scoring in hot, breezy conditions.
While Schwartzel and a few others flourished on the challenging Nicklaus-designed layout, McIlroy huffed and puffed as he plummeted toward the bottom of the leaderboard after repeatedly leaking shots to the right.
"That's sort of been my big miss all of this year," McIlroy said after a round that included two birdies, six bogeys and his expensive double at the 12th.
"It cost me today, obviously. I hit a few shots, especially around the front nine, in the water and stuff. Yeah, it was just a bit of a struggle out there."
McIlroy, whose best finish on the 2013 PGA Tour was a runner-up spot at last month's Texas Open, missed the cut at last year's Memorial Tournament and knows what he needs to do in Friday's second round if he is to avoid a repeat.
"I need to shoot something like 66 or 65 to probably make the weekend," said the 24-year-old Northern Irishman, a six-times winner on the U.S. circuit who already has two major victories to his name.
"That will be the target tomorrow, try to shoot the lowest score possible and see what happens."
Woods, who has triumphed four times in just seven starts on the 2013 PGA Tour, rued poor putting on increasingly crusty greens as he mixed five birdies with four bogeys.
"It was probably the highest score I could have shot today," said the world number one, who clinched last year's title by two shots. "I didn't make much. Hopefully tomorrow I can make a few more putts.
"The greens were a little bit tough this afternoon, a little crusty and a little bumpy. I don't think there were too many low scores this afternoon."
Woods finished two strokes better than Masters champion Adam Scott of Australia and three-times major Ernie Els of South Africa, who played alongside Schwartzel in a high-profile grouping.
Chinese sensation Guan Tianlang, who at 14 became the youngest player to make the cut in a major at the Masters last month, bogeyed two of his last three holes for a 72 while England's former world number one Lee Westwood returned a 76. (Reporting by Mark Lamport-Stokes; Editing by Frank Pingue)