Laszlo Cseh underlines form with personal best in 100m butterfly

Cseh, who won Hungary's first world 200m butterfly title on Wednesday to become the oldest champion in the distance, sliced 0.54 seconds off his own 100m butterfly personal best to secure a semi-final berth as top qualifier.

The 29-year-old's time of 50.91 seconds was the first time a swimmer had dipped below 51 seconds since Michael Phelps' gold at the London 2012 Olympics.

Tom Shields of United States posted 51.09 seconds to qualify second fastest, while South Africa's world champion Chad le Clos finished eighth.

"This result was very surprising for me," Cseh told reporters. "I hope I will be faster still in the semi-finals. In Barcelona (in 2013) I finished second but I hope to fight for the gold".

Meanwhile, world and Olympic champion Katie Ledecky of the United States progressed serenely into the women's 800m freestyle final.

The 18-year-old, bidding for four individual freestyle titles, recorded eight minutes 19.42 seconds to secure a final berth ahead of Australia's Jessica Ashwood who touched 2.75 seconds behind. Denmark's Lotte Friis qualified third.

The distance events have really improved and it's an exciting time to be around them," said Ledecky ahead of Saturday's final. "I always like to finish on a great note and come away with a good feeling."

France's Olympic champion Florent Manaudou edged rival Nathan Adrian of the United States out of a top qualifying berth in the men's 50m freestyle heats.

Manadou recorded 21.71 seconds in the one length dash, with Adrian touching 0.02 seconds down.

Swede Sarah Sjostrom's form continued into the women's 50m butterfly as she led a Nordic one-two into the semi-finals.

Sjostrom, who won women's 100 butterly gold on Monday, posted 25.43 seconds - exactly one second outside her world record - to top qualifying ahead of Danish rival Jeanette Ottesen.

The Dane, who won silver behind Sjostrom in the 100m butterfly, advanced through in 25.51 seconds.

Hungary's Katinka Hosszu put aside the disappointment of failing to qualify for the women's 200m butterfly final by posting the fastest qualification time in the 200m backstroke.

The Hungarian all-rounder recorded 2:07.17 seconds to finish 0.67 seconds ahead of Missy Franklin, of the United States.

In the men's 4x200m freestyle, Australia's quartet, featuring distance great Grant Hackett, posted 7:08.40 to top qualifying ahead of the USA and Great Britain.