Each man sits at 9-under-par 207 for the tournament, two strokes better than Soren Hansen (70) and three better than Miguel Angel Jimenez (70) and Carl Pettersson (74).
In reality, Karlsson should probably have owned the lead outright to himself entering Sundays final round. He played beautifully all day through the gusty winds which swept across Valencia, and through 16 holes, commanded a three-shot lead over the field.
However, costly bogies at the 17th and 18th saw his score inflate to 71 on the day. And it created a possibility of someone catching him.
Clarke did just that.
The Northern Irishman held on during an up-and-down round, in which he had recorded two birdies against three bogeys, for a three at the par-4 18th, giving him a 72 and a score of 9-under for the tounament.
Clarke is looking for his first win of the season on the European Tour. He won twice last year, taking trophies at the WGC World Match Play Championship and the Compass Group English Open, and finished second on the tours Order of Merit.
Karlsson is seeking his fourth career win in Europe. He has performed well in 2001, making top-10 finishes in three of his five starts ' including a runner-up showing at the Qatar Masters ' but he has not claimed a victory since the 99 Belgacom Open.
Of the 83 players in Saturdays round who survived the cut, just three managed to break 70 during Day 3. Stephen Gallacher, Christopher Hanell and Jean Hugo all carded 69s for the day.
Full-field scores from the Via Digital Open de Espana