Need the scores from any match played in this or any other tournament? Results
Llodra won his second career title in Adelaide, Australia last month.
Soderling defeated Gilles Simon of France 6-2, 6-1. The Swede also has two ATP titles. He and Llodra, both unseeded, will meet for the first time.
Llodra had to overcome 16 aces from Karlovic, whom Llodra broke only once. Llodra has yet to drop a set this week.
Soderling needed only 50 minutes to put away Simon.
"You don't really expect that in a tournament like this," he said of the ease of his win. "[Simon] is a player to beat. He makes no mistakes."
Soderling returned to the tour last week after a six-month layoff with a left wrist injury and reached the quarterfinals in Marseille.
If he wins on Sunday, Soderling will become the first Swede to claim the Rotterdam indoor title since Anders Jarryd in 1993.
All eight seeded players, including world No. 2 Rafael Nadal, exited in the tournament's opening two rounds.