Els began the day one shot behind overnight leader Charlie Wi of Korea and got off to a quick start with two birdies over the first three holes at the Emirates Golf Club. However, he ran into trouble four holes later with a double-bogey at the par-3 seventh.
'The flag was tight front left and there is a water hazard short of that and I guess I hit the wrong shot, the ball came up a little high, the wind got it, and it came up short,' said Els.
The two-time U.S. Open winner would not be deterred as he picked up back-to- back birdies from the ninth to climb to minus-10 around the turn.
Els added three more birdies on the back nine to pull away from Fasth and position himself to win for the second time in as many weeks.
'I'm very satisfied with the position I'm in,' said Els. 'I just went out there and played the golf course and played it fairly well so I'm happy and enjoying myself. I said before I started that I enjoy the course and that's what I'm doing, I'm just playing my own game.'
Els went winless on the European and U.S. PGA Tours last year, but in a reversal of fortunes the South African has put himself in position to post victories on both circuits in consecutive weeks.
Els has already earned his first European Tour victory since 2000 at the Heineken Classic last month. Last week he outlasted a late challenge from Tiger Woods to win the Genuity Championship for his first title in America in a year and a half.
Fasth had challenged Els earlier in the round with four birdies through the 13th hole. The Swede did not pick up any ground beyond that point and distanced himself further from Els after dropping a shot at the par-4 17th, the same hole he bogeyed in round two.
'I've been playing steadily for three days now and I've made only two bogeys coming off wedges at the 17th which just felt stupid,' said Fasth. 'I should be very happy because I played great but I'm afraid it was a little bit of a disappointing 69 today.'
Another Swede found himself near the top of the leaderboard after the third round. Carl Pettersson produced a bogey-free, 7-under 65 for the low score of the day. He stands at 8-under 208, five shots behind Els.
A group of four players finished together at 6-under 210, including Andrew Oldcorn (69) of Scotland. He was joined by France's Raphael Jacquelin (68) and the English duo of Paul Broadhurst (71) and John Bickerton (73).
Defending champion Thomas Bjorn carded a 1-under 71 to finish alongside England's Greg Owen (70) and Sven Struver (70) of Germany at 5-under 211.
Wi, who posted a 67 to take the lead in the second round, struggled to a round of 77 that included a double-bogey at the ninth. He dropped to 4-under 212 to join a large group of players nine shots off the lead.
Full field scores from the Dubai Desert Classic