The silver medallist in 2021 touched out in 23.85 seconds, just 0.04 slower than the Olympic record set by Australian Emma McKeon in Tokyo.
The world record of 23.61 was set by Sjostrom at the World Championships in Fukuoka, Japan, last year.
"I've never been that fast in a prelim swim at this kind of competition, like Olympics or world champs, so I'll take that," he told reporters.
"For the 50 I usually need a few races before I really get the extra little speed. I definitely think I will be a little bit faster once I go through."
Sjostrom won gold in the 100-metre freestyle on Wednesday and is chasing a sprint double at the La Defense Arena.
The pool has so far produced only one world record although plenty of Olympic records have tumbled.
The 50m semi-finals are on Saturday evening with the final on Sunday.
"I'm not going to force it and try to go for a world record," said the Swede.
"I'm just going to try to be as smooth as I can and then I'll see if I go 23.5 or 23.9 or 8, I don't know. Hopefully I will be a little bit faster than I was in the morning, that would be great preparation."
Sjostrom is also in Sweden's 4x100m medley relay team, with that final the last race on Sunday to close out the swimming programme.
