Andy Murray pulled out of the Monte Carlo Masters on Tuesday.
The eighth seed was due to play Frenchman Marc Gicquel in a second round match, but failed to recover from the back problem that forced him to retire during a doubles game on Monday.
"The pain was getting worse and worse by the end of the (first set) tiebreak. I could barely stand up," the Scottish world number 10 told a news conference.
"It's better today than it was yesterday, but I still find it hard to walk. It's a little bit frustrating."
The Briton will return home on Wednesday for a scan.
He said: "I guess I'll get a diagnosis from this and I'll know exactly what the problem is and how bad the inflammation is and how long it's going to take to go away."
Murray's place in the draw will be taken by Spanish lucky loser Guillermo Garcia Lopez.
