Budapest - Freestyle queen Katie Ledecky on Friday cruised towards her fifth gold at the world championships as the fastest into the women's 800m final in Budapest on Friday.

Despite only turning 20 in March, Ledecky is already the most decorated female swimmer in world championships' history.

She earned the 13th title, spanning three championships, by anchoring the USA's quartet to victory in the women's 4x200m freestyle relay on Thursday.

Ledecky clocked eight minutes, 20.24 seconds in the 800m heats on Friday morning, the fastest into Saturday night's final as the defending champion. 

"It was fine, it went to plan," said Ledecky. 

"I just had to get my lane for tomorrow, move on and have one more swim," she said. 

"I'm excited. I always enjoy my last race at a meet, just giving it all and leaving it all in the pool."

Ledecky already has five medals at these championships - freestyle golds over 400m and 1500m, plus the 4x100m and 4x200m relays, as well as silver in the 200m freestyle. 

Fresh from victory in Thursday's 100m freestyle final, Caeleb Dressel, 20, was second fastest into the men's 50m freestyle semi-finals during a busy morning for the American.

Brazil's Bruno Santos was the fastest at 21.51sec with Dressel just 0.10 behind.

Australia's Cameron MvEvoy was the seventh fastest through at 0.44. 

"It was good, I felt strong," said McEvoy.

"It was about where I thought I could go physically, which is a positive, there is a lot to be happy about." 

Dressel was back in action less than half an hour later as the fastest qualifier in the men's 100m butterfly heats, blasting out in 50.08 -- a clear second ahead of his nearest rival. 

Singapore's Joseph Schooling, Britain's James Guy and South Africa's Chad le Clos all booked their place in Friday's semi-finals. 

Sarah Sjostrom, the women's 100m butterfly gold medallist on Monday, is the name to beat over 50m by clocking 25.25, the fastest into Friday's semi-finals.

Like Dressel, it was also the start of a busy day for the 23-year-old. 

She is the favourite in the women's 100m freestyle final on Friday having become the first women to cover the distance under 52 seconds last Sunday with a new world-record of 51.71.

However, she was far from happy with her 50m butterfly swim. 

"The time was good, but everything else was bad," said Sjostrom.

"I had planned to do the first 25 fast and then relax, but the start was bad, everything went wrong in the first 25 metres, then it got better towards the finish. 

"In the 100m free final, I aim to go under 52 seconds again, if I do that I think I will win it, but it will be impossible to break the world record again."

Japan's Rikako Ikee was the third fastest at 0.47 into the 50m butterfly semi-finals, while Australia's Holly Barratt was only 12th fastest at 0.66 back. 

Hungary's 'Iron Lady' - Katinka Hosszu - had plenty of noisy support at the Duna Arena as she clocked the second fastest time, 0.48 secs, into the women's 200m backstroke semi-finals behind Kathleen Baker of the USA who swam 2:06.82. 

Australia's Emily Seebohm was the third fastest at 1.12 back, while Canada's Kylie Masse, who broke the women's 100m backstroke record on Tuesday, was fifth fastest at 1.48 back. 

"I just have to keep my race together," said Seebohm. 

"I want to go faster and then see how we go, I just want to get through."