Australia's Karrie Webb, twice a winner of the British Open before it replaced the du Maurier Classic as the LPGA's fourth major championship in 2001, shot a 2-under 70 to tie Michelle Ellis, Tina Barrett, Beth Bauer and Candie Kung for fifth place at 9-under 207.
Koch, the co-leader with Kung coming into the day, three-putted for bogey at the 17th but rolled in a 30-foot birdie putt at the closing hole to regain a share of the top spot. The 31-year-old Swede posted her third straight 4-under 68 at Turnberry's Ailsa Course and is in position to win her first major title.
'I just have to go out there and be aggressive,' said Koch, a two-time winner on the Ladies European Tour, who captured her breakthrough LPGA Tour title at last year's Corning Classic. 'To be in the last group on any occasion, but especially a major in Europe, I can't think of a better position to be in.'
Although she will take the lead into the final round of a major for the first time in her career, Koch is a seasoned veteran when compared with most of the players near the top of the leaderboard. Three players -- Gulbis, Bauer and Kung -- are LPGA rookies, and the average age of the top nine golfers through three rounds is just over 25 years old.
'It makes me feel old,' lamented Koch. 'There are so many young good players out there now, and they are not afraid to make mistakes, and maybe that's the type of golf you have to play out here.'
The lone player with a major championship pedigree among the relative unknowns is Webb, who is seeking her sixth victory in a major. With a win this week Webb would become the first player to achieve what has been dubbed the 'Super Career Grand Slam,' that is, she will have won the five different major titles available during her career.
The 27-year-old Aussie has 11 come-from-behind victories, all after starting the final round down two strokes or more. In fact, Webb's lone LPGA Tour triumph of 2002 came when she made up a five-shot deficit over the final 18 in Rochester.
She began the third round three shots off the pace but made four birdies in a front-nine 32 that included a 30-foot putt at the par-3 sixth. After following a birdie at the 11th with a string of four straight pars, Rosales sank a nine-footer for birdie at the 16th to tie Koch for the lead at 11-under.
At the par-5 17th, Rosales came up just short of the green with a 5-wood then chipped up to six feet. She made the birdie putt to seize the outright lead, then parred the 18th to match the best round of the tournament. Her 65 was also a career-low, besting the 66 she shot in the third round of last month's Jamie Farr Kroger Classic.
'I am really surprised,' Rosales said of being in the lead for the first time in her career. 'But I felt really good this week hitting my drives in the fairways where I want to be and hitting as many greens as I have.'
Rosales is no stranger to winning. A five-time Philippine Ladies Amateur champion, she moved to the U.S. to play college golf at the University of Southern California, where she won the NCAA individual championship as a freshman in 1998.
She joined the LPGA Tour in 2000 and made the cut in 15 of 23 events. Though she made only 12 of 26 cuts in 2001, Rosales recorded a tie for third at the Office Depot tournament for her career-best finish.
Rosales is currently 29th on the 2002 LPGA Tour money list with three top-10 finishes, including a tie for seventh in the U.S. Women's Open.
'I'm excited,' said Rosales. 'Not nervous, though. I am looking forward to getting experience tomorrow and we'll see what happens.'
Koch, playing in the final pairing four groups behind Rosales, racked up a trio of birdies over the first seven holes to take a two-shot lead at 11-under. Though Koch's lead evaporated as she parred holes eight through 14 while Rosales was putting the finishing touches on her dominating round, the Swede regained a share of first place with a 12-foot birdie putt at the 15th.
Koch stumbled on the 17th, where her third shot checked up 25 feet short of the pin. She left her birdie attempt four feet short then missed the par putt to drop one back.
Koch made up for the three-putt bogey by draining a 30-footer at the last that pulled her alongside of Rosales.
'I think the key for me tomorrow is to make sure I stay aggressive,' said Koch, who already has a career-high seven top-10s this season. 'That's the only way you win a major, or any golf tournament for that matter.'
Among Koch's top-10s results are a tie for fifth at the Kraft Nabisco Championship and a tie for sixth at the LPGA Championship. She shared 37th place in the U.S. Women's Open.
Gulbis, who leads No. 2 Bauer and No. 3 Kung in the LPGA Rookie of the Year standings, capped a flawless 67 with a chip-in birdie at the 18th.
'I didn't really have anything to lose coming in here,' said Gulbis, who at the age of 19 years, seven months and four days, would become the youngest player to win a major championship in LPGA history. 'I'm just trying to have fun and I have had a great week.'
The 22-year-old Marti, a second-year player on the Ladies European Tour, carded a 3-under 69.
Kung, who grabbed the first-round lead with a 65 and was tied with Koch after 36 holes, suffered bogeys on 11 12 and 13 to slip back to 6-under-par. However, she recovered with birdies at 14, 16 and 17 en route to a 71.
Full-field scores from the Weetabix Women's British Open