Adam Peaty did not take part in the mixed 4x100m medley relay heats on Friday morning, as Team GB began the defence of their Olympic title.
James Wilby swam the breaststroke leg as the British quartet qualified for Saturday’s final fifth-fastest.
Peaty, 29, tested positive for Covid-19 on Monday – a day after winning Olympic silver in the 100m breaststroke.
Missing the heats does not mean Peaty could not come in for the final on Saturday evening.
He could also compete in the men’s 4x100m medley relay which begins on Saturday morning, if fit enough and if the coaches wish to utilise him in that race.
Peaty did not swim in the heats as Britain took bronze in the mixed event at the World Championships earlier this year before coming in for the final, but he did when Britain won the Olympic title three years ago in Tokyo.
He has been in the pool in recent days during training sessions, raising hope that he will be fit to compete.
Wilby was joined by Kathleen Dawson, Joe Litchfield and Anna Hopkin as Great Britain finished third in their heat, behind the United States and China.
The USA were the fastest qualifiers, 0.44 seconds quicker than Australia who won the first heat.
Matt Richards, Tom Dean, Duncan Scott or James Guy, who won gold in the men’s 4x200m freestyle relay, could also come in for the final.