The teenager, part of a 16-strong Great Britain squad, finished 13th in the heats of the women’s 200m butterfly.
Stephens clocked 2:08.94 minutes to finish fifth in a strong first heat in Windsor.
The youngster, competing at world senior level for the first time, will swim again in the 100m butterfly heats on Saturday and could also be involved in the medley relay.
Meanwhile, her Plymouth Leander team-mate Felicity Passon, swimming for the Seychellois, finished 37th in the 100m freestyle event in 55.38 seconds and 40th in the 200m freestyle in 2:02.89.
There was a St Lucia national record in the men’s 200m freestyle. He clocked 1:53.79 as he finished 83rd in the heats.