Karlsson, who claimed his second victory of the season last week at the Players Championship of Europe, carded one eagle, nine birdies and two bogeys in his third round to reach 9-under-par 207.
He birdied the par-4 18th to complete his jump from 45th place overnight and was later joined by Colsaerts, who shot 73 but maintained the share of the lead he held following the second round.
The hangover from his win last Sunday wasn't fully realized by Karlsson until he opened with a 75 on Thursday. Since then, he has played 36 holes at 12 under.
'I made the mistake of playing on Tuesday, not realizing how much winning had taken out of me,' said Karlsson, who also won this year's Wales Open. 'The result was I shot 75 in the first round.'
Reflecting on the world's No. 1 player, Karlsson took solace in not being alone.
'[L]ook at Tiger Woods,' he said. 'He has this every time he plays, and what a lot of respect I have for him. When I was 4-over after six yesterday it didn't look too good, so it was a big challenge. I told myself to just keep patient, don't push too hard.'
The result was his record score at Kungsangen Golf Club and a one-shot lead alongside Colsaerts.
'Today I was happy to play so stress-free,' Karlsson said.
Colsaerts, meanwhile, took a step back in his quest for career victory No. 1. The 23-year-old Belgian posted just two birdies to go along with three bogeys in the third round.
Still, he has held at least a share of the lead following each round this week.
'I felt I could have done a lot better on the back nine,' Colsaerts said. 'I made a good save on the 17th, so maybe it evened itself out, but I felt I had my foot under the pedal.'
Jarmo Sandelin is alone in third place at 8-under-par 208 after shooting 66 on Saturday.
Fellow Swede Peter Gustafsson carded a 67 and shares fourth place with David Carter (70), co-second round leader Mark Roe (75) and Marc Warren (73) at minus-7.