Aaron Peirsol was the victim of the biggest shock of the World Championships so far when he failed to qualify for the final of the 100metres backstroke in Rome.

The American is the only man to have gone under 52 seconds for the event and won the last three world titles. Second at halfway in the semifinal, Peirsol miscalculated on the second length to find himself in fourth in 53.22sec and ninth overall -denied a place in the final by Britain's Liam Tancock.

Peirsol admitted he had made a major error and said: "I thought I was going faster. The result has surprised me. I am very disappointed - I completely misjudged my race."

Britain's Hannah Miley, who favours the 400m individual medley, was sixth in the 200m and said: "I am just over the moon with that."

Gemma Spofforth and Lizzie Simmonds qualified for tonight's final of the women's 100m backstroke.