Andy Murray withdrew from the Miami tournament (March 24-April 4), for which he had received an invitation, on Tuesday because of a groin injury.
The 33-year-old woke up on Saturday with a pain of undetermined origin that is making it difficult for him to walk.
"I have no idea what could have happened to me. It's getting better every day but not enough. I haven't trained since Friday," said the three-time Grand Slam winner, who has fallen to 119th in the world after several injuries.
Murray, who missed the Australian Open, has played just three matches on the main tour this year, only one of which he won. He has just reached the final of the Challenger tournament (2nd division) in Biella, Italy, where he lost to Ukrainian Illya Marchenko (212th in the world).