English and Karlsson still had 11 holes to play in the third round after the first two days were also interrupted by foul weather.
English, who won the FedEx St Jude Classic in June, finished off his second round on Saturday morning, adding three more birdies to his overnight score to finish with a nine-under-par 62, the lowest round of his career.
Karlsson, an 11-time winner on the European Tour, was one of only a handful of players able to sleep in, having finished his second round on Friday.
The former world number six continued his good form, also grabbing three early round birdies.
With no status on the tour, Karlsson was only included in the field courtesy of his top 10 finish in last week's McGladrey Classic, where he was on a sponsor invite.
The last of his wins in Europe came almost three years ago in the Dubai World Championship and he has slipped to 287th in the world but made 18 birdies in this tournament.
South African Rory Sabbatini (eight holes) and American Kevin Stadler (six holes) were tied for third place at 12-under while Spain's Alvaro Quiros (14 holes) and Americans Jason Bohn (nine holes) and Chris Stroud (eight holes) shared fifth at 11-under.
Defending champion John Huh sat 10 shots adrift at five under through 15 holes in a tie for 32nd.
(Reporting by Ben Everill; Editing by Julian Linden)
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