'Before this year I never thought I could win,' said Derksen. 'But I started believing in myself a bit more this year. Ernie was kind to make a bogey on the last yesterday so I thought, three strokes behind, I had a chance if I played well. I needed a bit of luck but that is what I got today.'
Derksen made a steady climb up the leaderboard on the Majlis Course at Emirates Golf Club and the turning point came on the back nine. He sent his second shot onto the fringe at the par-5 13th and drained a long putt for an eagle to reach 16-under and tie Els in the lead.
Meanwhile, Els hit his approach over the putting surface at the 12th and his third shot failed to reach the green. The South African ended up with a double-bogey at the hole to fall to 14-under and complete a four-shot swing.
'I only made one mistake today on the 12th,' said Els, who shot a final-round 69. 'I had 175 yards to the hole into the wind and it was just the first cut. I hit a seven-iron and just hit it long and it flew. It flew 200 yards and I had no shot from there.'
Els recovered with a two-putt birdie at the 13th and chipped in for birdie at the 15th to match Derksen in first. He had opportunities to regain the lead outright, including a near miss at the 16th and an eight-foot birdie try at the 17th.
Derksen had his nerves intact down the stretch with several safe plays and achieved the decisive birdie at the par-5 closing hole. He knocked his third shot four feet from the cup and made the putt to enter the clubhouse with the lead.
Els took to the 18th moments later, a hole that plagued him with bogeys on each of the two previous days. This time he avoided the water and sent his second shot over the green. The chip that followed, however, wrecked his chances at a comeback.
The reigning British Open champion was left with a 20-foot birdie try to force a playoff and, as an anxious Derksen watched, Els missed the putt.
'I travelled to Johannesburg and Singapore as first reserve but didn't get into the field. I only knew as I boarded the plane from Holland on Monday that I was going to play in Dubai,' said Derksen, a four-time Dutch Amateur champion. 'I knew the re-rank was coming up for Qualifying School graduates but now I don't need to worry about that and it's a wonderful feeling.'
For Els, the only two-time winner of the Dubai Desert Classic, his third European Tour victory of 2003 was in his grasp but slipped out of his hands down the stretch.
'I didn't hit a very good chip to say the least but I was concerned it was going to run away with me,' said Els of his play on 18. 'But what can you do. One guy comes out of the pack and beats you. He played well so it is one of those things.'
Ian Woosnam completed a fine week with a 3-under 69 to finish in a tie for third. Woosnam was joined by David Lynn and Alastair Forsyth at 14-under-par 274.
Thomas Bjorn, who captured this event in 2001, chipped in for birdie at the last to finish four shots off the pace at 13-under-par 275 along with Mikko Ilonen and Kevin Na.
Phillip Price and Thongchai Jaidee were one shot further back at 12-under-par 276 while Darren Clarke and Brian Davis followed at 11-under-par 277.