Els, the defending champion who has won this event four times overall, will face Vijay Singh on Saturday. Singh needed 38 holes to hold off Shaun Micheel on Friday and will now meet the world No. 2 in the semis for the second straight year.
In a return trip to England, British Open champion Ben Curtis disposed of Chad Campbell, 5 and 3. Masters winner Mike Weir was not as fortunate and came out on the losing end of a 5-and-4 drubbing courtesy of Thomas Bjorn.
Els played steadily in the morning at the Wentworth Club and used an eagle at the par-5 12th en route to a 2-up lead after the first 18.
The South African picked up steam in the afternoon and knocked his tee shot inside two feet at the par-3 fifth to win the hole and go 3-up. He followed that up with an 18-foot birdie at the sixth and another birdie at the seventh to open a commanding lead at 5-up.
Things got interesting on the inward half after Els bogeyed the 13th to lose the hole. Clark then won the 14th with a birdie and the 15th with a par.
At the par-4 16th, Clark dropped his approach within five feet of the hole and drained the birdie putt to win his fourth consecutive hole and pull within one.
Both players birdied the 17th to keep Clark's hopes alive but Els put the match away with a birdie at the last.
'You have to take your hat off to Tim Clark because he showed so much character coming down the stretch,' said Els. 'I had him 5-down before I played that tee shot into the bush on the 13th. Before then I felt in control and comfortable but I gave him a little breathing room and he totally responded.'
Clark took plenty away from the experience as he prepares to compete at the Presidents Cup next month.
'I'm proud of myself for the way I kept going and it is nice that at least I made Ernie work very hard in the end,' said Clark. 'I enjoyed being part of this tournament.'
Micheel was 2-up after the morning session and took a 3-up lead with an eagle at the par-5 fourth after his second shot stopped four feet from the hole.
Singh battled back with wins at the sixth and the seventh before pulling even with Micheel after a birdie at the ninth.
Micheel hit a bunker shot to five feet at the par-4 16th and ran home the putt for a birdie to regain a 1-up lead. The PGA champion found trouble on the very next hole, however, and sent his ball out of bounds twice before conceding the par-5 17th.
Singh and Micheel matched birdies at the 18th and pars at the first. Micheel found trouble off the tee again at the par-3 second and kicked his ball into a greenside bunker.
Micheel couldn't recover and bogeyed the hole giving the match to Singh.
'I'm really happy with the result,' said Singh. 'Any time you win a match, a long battle like that, you feel for the other guy. It was a long day and a tough day especially with the swirling winds and the greens were getting firm.'
Curtis began to put his match out of reach after a birdie at the par-5 12th in the afternoon gave him a 3-up lead over Campbell.
Campbell gave another hole away with a bogey at the 14th to fall 4-down and found further trouble at the 15th before conceding the hole and the match.
'I'm really thrilled to get through,' said Curtis. 'To be honest I haven't been playing very well of late and I have been working on a few things so I'm glad they worked out and really glad to be playing tomorrow.'
Bjorn was in complete control of his match with Weir as the Canadian never held a single lead throughout the day.
The Dane completed the victory with a birdie at the 14th and will move on to meet Curtis, who took the glory at Royal St. George's this summer after Bjorn's late collapse.
'I lost the Open and whoever I lost it to it wouldn't have made any difference,' said Bjorn. 'I lost it to Ben Curtis but it could have been Tiger Woods and it would have felt the same.'