Kazan - 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, defending his title, clocked 26.62 to shave 0.05 seconds off his own record he set at the 2009 championships in the era when the performance-enhancing neoprene suits were still permitted.
Britain's Peaty had swum exactly the same time of 26.62 at the European championships in Berlin last August, but the 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 resume their battle 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 (be) very tight."
Peaty, who was second fastest into the semi-finals just 0.06 seconds 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 while Australia's Christian Sprenger, who won silver in the event two years ago in Barcelona, missed the semis by just 0.03 seconds with 27.54.
This is the fifth time a world record has been broken in Kazan and it is the first time a men's record has been broken 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.
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 1 500m freestyle record in Monday morning's heats.