Els will face reigning U.S. Open champion Angel Cabrera, who held off Hunter Mahan, 2 and 1, in Saturday's other semifinal match at Wentworth Club. Cabrera advanced to the final for the first time in four tries.
The second-seeded Els did not grab the lead in his match until the 25th hole. Stenson took a 1-up lead thanks to an Els bogey on the second, and the duo matched birdies on four and 11.
Els squared the match with an eagle on the par-5 12th. Stenson rebounded with a birdie on 13 to regain the advantage, but he gave that up with a double-bogey on the par-4 15th.
The duo swapped birdies on 16 and 17, then Stenson birdied the 18th to lead 1-up into the second 18. Els squared the match with a 20-foot birdie putt on the 19th and he would not trail again.
The match remained all square until Stenson three-putted for bogey on the 25th, giving Els his first lead. The South African quickly pushed his advantage to 3-up with back-to-back birdie putts from inside 6 feet at the 26th and 27th.
Stenson, who won the WGC - Accenture Match Play earlier this year, responded with a 25-foot birdie putt on the 28th to get back to 2-down. The two both birdied the 30th.
'I struck the ball nicely on the front nine,' Els stated. 'I played actually quite good, but I hit a bad tee shot on 10 and it kind of stuck with me a little bit.'
Els parred the next three holes as his lead grew to 3-up as Stenson could not save par on the 33rd after finding a fairway bunker off the tee. Els two-putted for par on the 34th and when Stenson's 12-foot birdie putt missed, Els was heading to the final again.
'I grinded it out. I made the putts when I needed to and some were huge putts,' Els said. 'I kind of putted like Henrik did this morning. The putter saved the day in the end.'
Cabrera never trailed against Mahan, who bogeyed the third to fall 1-down. The duo traded wins on six and seven before Cabrera dropped in birdies on eight and nine to push his lead to 3-up.
Mahan took the 10th with a birdie, but Cabrera responded with a birdie on 12. Mahan again got back within 2-down when he birdied the 15th. Cabrera pushed his lead back to 3-up with an eagle on the par-5 18th.
Cabrera pulled away early on the second 18 as he birdied the 19th and Mahan bogeyed the 21st. Cabrera extended his lead to 6-up with an eagle on the par-5 22nd.
However, Mahan rallied with four birdies in the next six holes to drop his deficit from 6-down to 2-down. Cabrera got one back as he birdied the 29th.
'I knew it wasn't me. He came back and made birdies,' Cabrera explained through his caddie/interpreter. 'Then I knew it was up to me when I was 2-up.'
Cabrera held on to his 3-up lead as the duo halved four straight holes. Mahan was 3-down with three to go, but kept battling. He made an 11-foot birdie putt from the fringe on the 34th to extend the match.
After Mahan made a 6-foot birdie putt on the 35th, Cabrera drained a 5-footer for birdie to close out the match and advance to the finals.
'I am really delighted. I played some of my best golf today, especially in the afternoon,' said Cabrera, who was 12 under for 35 holes Saturday. 'I only had one bogey. Hunter came back really strong and it was a really, really tight match and I'm happy to win.'