South Africa's Cameron van der Burgh broke the 50m breaststroke world record in Tuesday morning's heats at the world swimming championships as his battle with Britain's Adam Peaty resumed.
Van der Burgh, who is defending his title, clocked an official time of 26.62sec to shave 0.05sec off his own record he set at the 2009 championships in the era when the performance-enhancing neoprene suits were still permitted.
Peaty had swum exactly 26.62sec at the European championships in Berlin last August, but his time has still not been ratified by swimming's governing body FINA.
Van der Burgh, the Olympic 100m champion, lost out to Peaty on the wall in Monday's world championship final over the same distance.
The rivals are set to battle again over the shorter distance in Tuesday night's semi and Wednesday's final.
"I am really happy after the bitter-sweet disappointment of last night, when I didn't get the luck of the touch. But I guess I had it today," said Van der Burgh.
"We are all so close, it's not going to be a one- or two-horse race, there are a lot of guys in the mix and it's going to very tight."
Peaty, who was second-fastest into the semi-finals at just 0.06sec slower than the South African in his heat, admitted he was confused as to who held the world record after Van der Burgh matched his Berlin time.
"It will be nice to get out there tonight with my full game head on to focus on the race," he said.
"I am fully capable of going faster than... my old, well, equal world record."
Slovenia's Damir Dugonjic (26.70) and Kevin Cordes of the USA (26.93) also posted fast times going into the semis.
Australia's Christian Sprenger, who won silver in the event two years ago in Barcelona, missed the semis by just 0.03sec at 27.54.
- Ledecky double -
This is the fifth time a world record has been broken in Kazan and it is the first men's record to fall at a world championships since 2011.
Sweden's Sarah Sjostrom broke the women's 100m butterfly record twice in the space of 24 hours en route to retaining her world title on Monday here.
Hungary's Katinka Hosszu broke the women's 200m individual medley record as she won gold on Monday night and Katie Ledecky of the USA bettered her own 1500m freestyle record in Monday morning's heats.
Ledecky carried her impressive Kazan form into the morning's 200m freestyle heats and the 18-year-old triple world record-holder was the fastest in a strong field.
She has set herself a punishing schedule and will have just 20 minutes between the 1500m freestyle final and her 200m semi-final in Tuesday's evening session with the 800m freestyle heats still to come on Friday.
Fresh from breaking the six-year-old 200m IM world record on Monday, Hosszu was second fastest in the heats at 0.50sec behind Ledecky, with reigning world champion Missy Franklin of the USA at 0.60 back.
"I slept well last night, the 200m is a different animal -- it's more of a sprint for me," said Ledecky.
"Katinka is swimming really well, there are a lot of fast European girls in there as well as Missy Franklin in there so it's going to be a good race."
- Sun sixth fastest -
Hungary's Laszlo Cseh is on course to win his first worlds gold for a decade having qualified fastest into Tuesday's 200m butterfly semi-finals in a time of 1min 53.71sec.
China's long-distance expert Sun Yang was sixth fastest into the men's 800m freestyle final on Wednesday as he prepares to try to win the men's 200m final on Tuesday night for the first time.
Connor Jaeger of the USA was the fastest from the 800m heats in seven minutes 44.77secs while Australian teenager Mack Horton, who swam the year's fastest time coming into Kazan, was fifth quickest at 2.31secs back.
After retaining his 400m freestyle world title on Sunday, Sun is hoping to leave Kazan as the owner of the 200m, 800m and 1500m golds.