Hungary's Laszlo Cseh has expressed his shock after swimming sub-51 in this morning's 100m butterfly heats at the World Championships in Kazan.

The 29-year-old, who won gold in the 200m butterfly final earlier this week, finished heat six in 50.91s to qualify fastest for tonight's semi-finals.

"When I saw the time I thought, 'Jesus Christ, under 51 in the morning, no way'," Cseh told SwimVortex. "It was a good start and then in the second 50 I pushed because you have to push anyway. A best by half a second."

Cseh's swim is the first time since Michael Phelps at London 2012 to clock a sub-51 time.