'I don't think I'll win because I'll get nervous and stiff,' said Katayama.
Whether he's at the top through four rounds or not, the Japanese player is the man to beat entering the weekend. Katayama, who wears a cowboy hat on the course, shot a 5-under-par 66 on Friday to take a four-shot lead at 11-under over Scot Andrew Coltart and Northern Ireland's Darren Clarke.
Coltart was a co-leader after the first round, but slipped into a share of second after a second-round 71. Fellow overnight co-leader Edward Fryatt is in a tie for fourth following a day-two 72. Masters champion Vijay Singh is ten shots off the lead after carding a 2-over-par 73.
Katayama sank a 40-foot birdie putt on the first hole, and then added four more birdies on the front nine before parring the entire backside.
'I am surprised to have shot 65-66 because my swing is mediocre at the moment,' Katayama said.
Katayama has reason for his pessimistic outlook. Only two Japanese players - Jumbo Ozaki (three times) and Tommy Nakajima - have won Japan's biggest event in its 26-year history.
Past champions include Seve Ballesteros, Johnny Miller, Tom Watson and Ernie Els. Last year's winner, Thomas Bjorn, withdrew before the second round citing a pinched nerve in his back.