In the end, it was Jamie Donaldson that grabbed the 54-hole lead with four birdies in his last six holes.
Donaldson finished his day at Crans-sur-Sierre with a six-under 65 to take a one-stroke lead after 54 holes. He finished three rounds at 14-under 199 and will duke it out with Westwood on Sunday for his first European Tour title.
"It will be awesome going up against two of the best players in the world at the moment," Donaldson said.
For the nearly eight years Donaldson has played on tour, he's never led after 54 holes. He's notched only two runner-up finishes in his career, with the most recent being at the 2009 Scandinavian Masters in Sweden.
Westwood had a flawless afternoon with seven birdies for a day's best of 64 to vault into second place at 13-under. He'll challenge Donaldson with his vast experience, having 21 career European Tour victories, 12 of which came when trailing after three rounds.
"I played only six holes the last two weeks, which is not really the right preparation," said Westwood, who is battling a head cold and recently vacationed in Barbados. "But at some point in the year you have to take a break and recharge your batteries."
McIlroy had an excellent chance at heading into the final round with the lead, but a bogey-bogey stretch from the 16th left him in third at 12-under. Still, after a round of four-under 67, McIlroy, who held a share of the lead after two rounds, will have a good chance at his first win since his impressive U.S. Open performance.
"I'm still in there with a chance, and I'll be looking for a fast start and putting a bit of pressure on the boys," McIlroy said. "I feel the way my game is there's a 61 or 62 out there."
Donaldson, one of the four 36-hole co-leaders, got off to a nice start with three birdies in the first seven holes, but McIlroy drove to the green on the short par-four seventh and putted for eagle from 30 feet to move to 12-under for a one-shot lead.
After Donaldson, one shot back at that point, bogeyed the ninth, it appeared as though it was a day for McIlroy and Westwood, who caught McIlroy with a birdie at the 14th.
Westwood also birdied the 15th, but McIlroy matched the feat with birdies at both par-fives to stay one ahead for the time being.
Meanwhile, it was Donaldson -- in the final grouping -- who began to make a run at the 13th. He birdied three straight from there to match both Westwood and McIlroy at 13-under after McIlroy had bogeyed the 16th, and McIlroy's bogey at 17 dropped him off the pace.
"It was not the greatest way to finish," McIlroy said.
After Westwood parred the remaining three holes, Donaldson's birdie at the 17th was good enough to provide him the lead heading to Sunday.
Donaldson has had a solid year in 2011 other than his lack of a win, a streak that has plagued him for 237 European Tour events. He posted three top-10s in a four-event stretch starting with the China Open and has missed only one cut this year in 17 tournaments -- at the PGA Championship, his first major since 2006.
Thomas Bjorn, last week's winner at Gleneagles, shot a five-under 66 and shares fourth with 36-hole co-leader Gary Boyd (68). Martin Kaymer (68) headlines a pack at 10-under that includes Keith Horne (65), Soren Hansen (66), Damian Ulrich (67), Stephen Gallacher (67) and Danny Willett (68).
NOTES: All four par-fives on the course played under par on Saturday. Seven holes in all played under par...Defending champion Miguel Angel Jimenez (67) shares 15th at eight-under 205...Simon Dyson, who shared the lead heading into the day, stumbled to a 74 and tripped to a tie for 34th.