That would leave the Memorial with six of the top 10 players from the world ranking. Tiger Woods, recovering from April 15 surgery on his left knee, did not enter last week and Adam Scott decided not to play.
Singh, who won the Memorial in 1997, said he injured a muscle in his ribs upon arriving in England last week for the BMW Championship and felt pain while warming up on the range ahead of the first round.
Tournaments officials were trying to contact Els' managers after learning from his Web site that he wasn't coming.
"I have a week off now before my next tournament, the Stanford St. Jude Championship, start on June 5," Els wrote Monday. "I haven't made any firm plans yet, but I will probably stay here at Wentworth with the family."