After starting fast in his return to competitive swimming,
Racing at an Arena Pro Swim Series meet in Mesa, Ariz., the 18-time Olympic champion scratched from the 100-meter backstroke in the morning, then failed to qualify in the 400 freestyle a few hours later.
His subpar performance followed an impressive win in the 100 butterfly on Thursday.
"Probably some things to fix, but I think it's a good starting point," he said after that race.
Phelps just finished a six-month suspension stemming from a DUI arrest last fall. He has confirmed his intentions to try for the 2016 Rio de Janeiro Olympics.
The schedule in Arizona has him listed in two events on Saturday.