Kazan: More world records tumbled in the Kazan pool in Wednesday`s heats as the mixed 4x100m medley relay mark fell twice in back-to-back races.
The USA quartet of Ryan Murphy, Kevin Cordes, Kendyl Stewart and Lia Neal clocked three minutes, 42.33 seconds in the event`s third heat of the morning session.
The new world record came just minutes after Russia had swum 3:45.87 mins in the previous heat to better the old record of 3:46.52 set by Australia in Perth in January 2014.
"It`s a fun new event and I was very happy to get a world record...even if the Americans broke it in the very next heat," grinned Russia`s butterfly expert Daniil Pakhomov before the evening`s final.
World records have now fallen on nine occasions in Kazan and more threaten to go later on Wednesday when Britain`s Adam Peaty and Cameron van der Burgh of South Africa clash in the men`s 50m breaststroke final.
They both broke the 50m record on Tuesday when Van der Burgh lowered his own mark in the morning`s heats by clocking 26.62secs, only for 20-year-old Peaty to come out and break it with 26.42 in the semi-finals.
Australia`s Emily Seebohm brushed off tiredness from her gold medal exploits to storm into the women`s 50m backstroke semi-finals.
The 23-year-old led a golden night in the Kazan pool on Tuesday by winning the women`s 100m backstroke final before Mitchell Larkin won the men`s race as Australia enjoyed their best single day at a world championships for a decade.
Seebohm was the third fastest qualifier into Wednesday`s semi-finals, but admitted she had not felt at her best before the race.
"I didn`t get to bed until 1230am last night and I didn`t feel fantastic when I woke up this morning, but I`m beginning to brighten up a bit now," she admitted after winning her first individual world title.
"I loved every minute of last night and I wish I could do it all again.... well, maybe not the race, but everything which came afterwards."
Her team-mate Cameron McEvoy was the second fastest into the 100m freestyle semi-finals at 48.33sec, just behind China`s Ning Zetao who clocked 48.11secs, with Olympic champion Nathan Adrian of the USA only seventh fastest through at 48.61.
"It was a really controlled race. I wanted to make sure I was in a strong position for the semi-final," said McEvoy, who came last in the men`s 200m freestyle final on Tuesday.
"I was still feeling the effects of the 200m final the night before, it`s standard to feel a bit flat in the morning, but finishing last in the 200m hasn`t affected my confidence."
Adrian was fourth in his heat and was just happy to be in the semis.
"That was slower than we all expected it to be," said the 26-year-old.
"There are no worries, but we`ll see how it goes tonight."
Speed was no problem for team-mate Ryan Lochte in the 200m individual medley heats after blazing into the semi-finals as the fastest qualifier in his bid to win the world title for the fourth consecutive time.
The 31-year-old, who has won 15 world championship medals in his career, made up for the disappointment of fourth in Tuesday`s 200m freestyle final by clocking 1:57.90 mins despite gliding into the wall.
"Lst night wasn`t the best for me, but I`ve just drawn a line under it and moved on," said the 31-year-old Lochte.
"I woke up this morning, had a good warm-up and that was a good solid swim for me."
Hungary`s `Iron Lady`, Katinka Hosszu who broke the six-year-old women`s 200m IM world record on Monday eased through the 200m butterfly heats as the sixth fastest behind China`s Zhang Yufei.
Hosszu is racing in six events here in Kazan and having already retained her 200m IM world title, she will race in the 200m freestyle final on Wednesday night before her 200m butterfly semi.
"I`m looking forward to tonight, I have the 200m free which is what I am focussing on, I slept pretty well and I`m feeling good."