The 23-year-old smashed the world record in Sweden's lead off leg during the 4x100m freestyle relay with 51.71 seconds.
Sjostrom's silky smooth stroke earned her a time of 53.01 seconds, with Denmark's Pernille Blume posting 53.13 seconds to qualify second fastest.
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American Simone Blume, the Olympic champion, enters the semi-finals later on Wednesday as third seed.

Sjostrom eases into women's 100m freestyle semis

“It’s nice to be back after two days off," Sjostrom told reporters.
I have the speed you need from the start so there is no point in letting the speed go.
Olympic champion Ryan Murphy of the United States clocked one minute 56.11 seconds to head qualifying into the semi-finals of the men's 200m backstroke.
Hungarian Peter Bernek was rewarded for a blistering first 100m in qualifying second, 0.42 seconds behind. Defending champion Mitch Larkin of Australia edged into the semis in 13th.
Bernek, roared on by his home crowd, said: "I didn't expect to have pains after 150m, but when I looked at the scoreboard I could see I was within national record time."
Asia led a one-two into the women's 4x200m freestyle final. China edged Japan as top seed with seven minutes 51.75 seconds, with the U.S. and Australia also set to challenge for gold in the finals.
Molly Renshaw of Britain hopes to follow in the footsteps of double world champion Adam Peaty after she topped qualifying in the women's 200m breaststroke heats.
Renshaw, 21, recorded two minutes 24.03 seconds ahead of America's Lilly King, who won the 100m title this week.
"I've been on the scene since 2011 so I've had a lot of years to learn," Renshaw said.
I've always been involved in racing but now I'm ready to step it up and reach podiums and finals.
The Peaty effect was also evident in the men's 200m breaststroke heats as Ross Murdoch of Britain showed his credentials by going second fastest.
Murdoch was the last swimmer to beat Peaty over 100m when the Briton pipped him at the British Championships in 2014.

Murdoch produces solid swim to qualify for men's 200m breaststroke semis

In Murdoch's preliminary heats, top spot went to Russia's Anton Chupkov who touched in two minutes 08.23 seconds, while world number one Ippei Watanabe of Japan looked sharp enough in qualifying third.
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