RIO DE JANEIRO (AFP) - Michael Phelps launched his assault on a fourth successive Olympic 100m butterfly title on Thursday but added an awkward complication to his schedule after a rare miscalculation.

The world record holder has remarkably won the event at the past three Games but was peeved after failing to qualify for the evening's second semi-final and give himself extra breathing space after his defence of the 200m individual medley.

Like Phelps chasing a fourth Rio title, golden girl Katie Ledecky qualified fastest for the women's 800m freestyle final as she aims to become the first woman to sweep the 200m, 400m and 800m free since Debbie Meyer in 1968.

Phelps, who has already captured three Rio golds to take his record Olympic title haul to a staggering 21, clocked 51.60 seconds behind heat winner Joseph Schooling of Singapore (51.41sec), Hungary's European champion Laszlo Cseh (51.52sec) and fellow American Tom Shields (51.58sec).

"I screwed it up," said Phelps, who now faces a tough turnaround of just over half an hour between races. "I try not to even think about it because then I just get focused on that.

"I'm fine with that (time) but I'd like to have those seven or eight minutes in between."

World champion Chad le Clos, runner-up to Phelps in London four years ago, was seventh fastest as he looks to avenge his crushing defeat by the American in Tuesday's needle 200m fly.

Once again, Ledecky was in a league of her own.

"I try not to think about the history," said the American phenom after posting a Games record of 8:12.86.

"I know the significance of the 2-4-8, I would love to join Debbie on that."