Toms blew a seven-stroke margin Saturday, and Wi took the lead with a 32-foot birdie putt on the par-3 16th hole. At 13 under after a 4-under 66, Wi had a one-stroke edge over Toms -- and the 54-hole lead on the PGA Tour for the first time in his career.
"I'm very pleasantly surprised. I played really well today and David didn't," Wi said. "It's such a crazy game. I don't know what to say."
Soon after weather delay of 1 hour, 20 minutes, Wi's tee shot at No. 16 landed on the back side of the green and Toms pushed his shot right into the rough. Wi holed the birdie putt before Toms' 16-foot par chance just missed for a two-stroke swing that changed the top of the leaderboard.
"Tough day overall," said Toms, who shot 74 after building his huge lead with bogey-free 62s.
Toms opened with a birdie Saturday, but had three bogeys in a five-hole stretch while Wi got started with consecutive birdies.
Even worse for Toms was a three-putt from 7½ feet for double bogey at No. 14, where Wi chipped to 12 feet to save par before the delay.
"Until then I was still three behind him, so I wasn't even thinking about the lead or anything," Wi said. "Then it's 'Wow, I'm only one shot behind him.' I knew it was getting a lot more interesting than how I envisioned when I started the day."
Wi made his 100th cut in 147 PGA Tour events this weekend, but the 39-year-old South Korean has never won.
The 44-year-old Toms is a 12-time winner, but is looking for his first victory in more than five years. He is coming off a playoff loss to K.J. Choi last weekend at The Players Championship.
While Toms now has another disappointment to overcome, at least he still has one more round to play at Hogan's Alley.
"I'm right where I set out to be when I started this week," Toms said. "I certainly would like to be sitting here with a 10-stroke lead and trying to break some record or something like that. It's all about getting in position and see how I do. I did well last Sunday with a chance, but didn't quite get there."
John Senden, who began the third round with Wi in a quartet of players seven strokes back, shot even par and was third alone at 9 under. Stuart Appleby (67), Paul Goydos (67) and Mark Wilson (71) were 8 under.
After blasting out of a frontside bunker at the par-4 14th, Toms three-putted for his first double bogey in a stretch of 343 holes at Colonial. That coupled with Wi's impressive up-and-down at the same hole cut the gap to one stroke.
Before Toms and Wi finished No. 15, play was stopped because of an approaching storm. Only a little bit of rain fell before play resumed and both made their par putts.
Toms got to 17 under with his opening birdie Saturday at the 565-yard first hole, chipping from just short of the green to 6 feet. Then came a couple of bad bounces and three bogeys.
His approach at the par-4 second hit on the front of the green but rolled back into the greenside bunker. He blasted to about 19 feet and two-putted for his first bogey in 38 holes.
"From there, he kind of lost the momentum a little bit, but I didn't think I was going to be able to close the gap like I did," Wi said.
After missing the fairway left at No. 3 and hitting a low liner approach that stopped just short of the green, Toms pushed a 5½-foot par chance just right of the hole.
At the difficult 247-yard fourth hole, his tee shot landed in the frontside bunker so deep that the 5-foot-10 Toms' head was barely visible from the back of the green when he blasted to 9 feet. When he made that putt, he had a slight fist pump that was more relief than celebration.
But Toms got another bad break when his approach at No. 6 rolled off the right side of the green. He chipped 9 feet past the hole and couldn't save the par, and walked away with holding both hands out.
"I felt like after the first six holes, I was just hanging on," he said. "Trying not to make a mistake rather than going out and playing great. That's probably what that big lead does for you. ... I really never felt nervous. I just didn't fell as confident out there today. I just didn't feel in control, and that's a bad feeling."
After carding 31s on both nines on each of the first two rounds, Toms finished the front side Saturday at 2-over 37. He had already missed five greens, matching his total for first two rounds.
Still, at that point, Toms still was 14 under with a three-stroke lead over Wi and Wilson, playing in the group ahead.
"It can happen fast on this golf course, you can go either way quickly," he said. "There are some birdie holes if you're playing great and there's a lot of bogeys out there to be had if you're not."