Second-round co-leader Justin Rose notched a 3-under 69 to share second place with K.J. Choi (68) and Fred Couples (68) at 10-under-par 206.
Lurking one stroke behind that trio is three-time Memorial winner and World No. 1 Tiger Woods, who posted a 5-under 67 Saturday. He shares fifth place with Stephen Ames (70) at minus-9.
Couples and Els seemed to feed off each others solid play throughout the round. Els, the world No. 3, got off to a good start at Muirfield Village. He birdied the par-4 second, then notched back-to-back birdies from the fifth to get to 9 under.
In the process, Els took over the lead. He settled down with four straight pars, while Couples caught fire. The University of Houston alum chipped in for his first birdie at the seventh, after opening with six straight pars.
Couples rolled in an 18-footer for birdie at the next to get to minus-8. After a par at the ninth, he dropped his second shot within 6 feet to set up his third birdie, one that gave him a share of the lead.
Couples then posted four straight pars, while Els answered with a solid stretch of golf. After a poor drive at the par-5 11th, Els knocked his third shot to 15 feet and he sank that birdie to tie Choi in the lead. Choi had climbed into the lead thanks to five birdies and a bogey over his first 10 holes.
Els parred the 12-14 to remain at 10 under and in the lead as Choi fell off the pace with bogeys at Nos. 11 and 13.
The South African moved to 11 under with a birdie at the par-5 15th, which was matched by Couples. Els hammered an 8-iron to within 7 feet of the cup at the 200-yard par-3 16th. He calmly sank that putt to move to minus-12, three clear of Couples who bogeyed the same hole.
Els parred in to remain at 12 under.
'It was a good day. I put myself in good position most of the day and I hit some good iron shots, and made some putts and saved some good pars,' said Els, whose best here was tie for second in 2000. 'I've got a two-shot lead, but it doesn't mean much.
'It means that I'm playing well, doing something right. I've just got to go out there and play golf tomorrow. I need to try to stay aggressive.'
Couples holed out from 70 yards for birdie at the 17th to get within two of Els. He parred the last to share second place. For the tournament, Couples has chipped in for birdie three times to go along with that hole-out birdie.
'I hung in there, hit the ball really, really well and had a good day,' said Couples, who won here in 1998. 'If anything, tomorrow for me personally will be a great day. I've been working on my game and it held up today. I'm definitely looking forward to tomorrow, I have a great shot at winning. It's just another great round of golf to be played at Jack's (Nicklaus) course.'
Meanwhile, Choi recovered from his troubles at 11 and 13th with consecutive birdies from the 14th to get to 10 under. He faltered to his fourth bogey of the day at the 16th, but managed to erase that mistake with a birdie at the next to share second place.
Rose recovered from a double bogey at the third to climb back into contention. He birdied the second and fifth, to head to the back nine at 7 under. He picked up birdies at Nos. 10, 14 and 17 to move into his share of second place.
By the way, Woods managed to remain among the leaders by hitting 79 percent of his fairways and needing only 24 putts to complete his round. He stumbled out of the gate with a bogey at the first, but came back with birdies at three and five to move to minus-5.
Woods nearly holed his pitch shot for eagle at the par-5 seventh, but settled for a tap-in birdie. After a pair of pars, Woods stuck a 9-iron inside 3 feet of the hole at the 10th for his fourth birdie of the round.
The 28-year-old dropped his third shot to the par-5 11th inside 8 feet and sank that birdie try to get to 8 under. He left his third shot in a greenside bunker at 14 and nearly holed his fourth from the beach, but walked away with bogey.
As he often does, Woods made up for the error with an up-and-down birdie at the next. He capped his round by rolling in a birdie putt from just over 20 feet at No. 17 to get within three strokes of Els.
'I hit better golf shots, and also I got some pretty good breaks, as well,' said Woods. 'For instance, on No. 2 today, the ball should have been in the hazard, but it wasn't. I had a swing, I had a difficult shot, but at least I had a swing, and I made par where that should have been a bogey. Bogey-bogey is not the way you need to start on a Saturday afternoon.'
Defending champion Kenny Perry (66) and reigning British Open champion Ben Curtis (73) share seventh place at 6-under-par 210. Retief Goosen and Jay Haas are one stroke further back at minus-5.