[RIO DE JANEIRO] Michael Phelps launched his assault on a fourth successive Olympic 100m butterfly title 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 200, 400 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 (51.41), Hungary's European champion Laszlo Cseh (51.52) and fellow American Tom Shields (51.58).
"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," he added. "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 eight minutes, 12.86 seconds. "I know the significance of the 2-4-8, I would love to join Debbie on that."
The world record holder, who won the event as a fresh-faced 15-year-old in London, is the overwhelming favourite after storming to victory in the 200 and 400m before adding gold in Wednesday's 4x200m free relay.
Hungary's Boglarka Kapas was a distant second in 8:19.43 with Britain's Jazz Carlin third in 8:19.67.
Olympic and world champion Florent Manaudou was only fourth quickest among the speed demons in the men's 50m freestyle heats, the giant Frenchman clocking 21.72 seconds.
American Anthony Ervin got off to a solid start to his bid to become the oldest swimming champion in Olympic history at 35-years-old, going third fastest in 21.63, two hundredths behind countryman Nathan Adrian.
"I'm living in the moment, man," said the joint 2000 Sydney gold medallist.
"I'm not thinking about what I'm going to do other than how I'm trying to set myself up."
Adrian, the world silver medallist, qualified second fastest behind Ukraine's Andrii Govorov, the pace-setter with a 21.49.
Hungary's "Iron Lady" Katinka Hosszu topped the time-sheets in the 200m backstroke heats, clocking 2:06.09 as she bids for a fourth Rio title after completing the individual medley double and winning the 100m backstroke.
Reigning champion Missy Franklin was over three seconds slower, the American just bumped out of the top 10 by Australia's world champion Emily Seebohm.