Andy Murray (right) was in action with his elder brother

Murray damaged his back while playing in a doubles match alongside his elder brother Jamie at the same tournament.

"It's better than it was on Monday, but I still find it hard to walk - it's a little bit frustrating," he said.

The 19-year-old, who could miss three weeks, was set to meet Marc Giquel in the second round of the singles.

News conference: Andy Murray

Murray's place in the draw was taken by lucky loser Guillermo Garcia-Lopez of Spain.

The Murray brothers lost a first-set tie-break against Poland's Mariusz Fyrstenberg and Marcin Matkowski before Andy withdrew with the injury.

"The pain was getting worse and worse by the end of the tie-break. I could barely stand up," added the world number 10, who was seeded eighth for the tournament.

"I hit a shot and my back went into spasms and got worse and worse after every game.

"This is an important time of the year for me because I didn't play so well on the clay last year and I wanted to start off well this year.

"I've been playing way better in practice than I was on the clay last year, so I'm a little bit disappointed.

I'm annoyed about it as I was playing pretty well Andy Murray

"I'm going to go back home today and have a scan. 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 has suffered a series of niggling injuries in recent weeks.

A month ago, the Scot was well beaten in the Indian Wells Masters semi-finals after suffering a hip injury in the previous round, and he pulled his groin muscle in Miami two weeks ago.

Murray said he blamed his punishing schedule for his run of injuries.

"I'm annoyed about it really as I was playing pretty well and my groin is feeling fine now," he said.

"This is where the schedule is a problem I think, and not just because I got hurt.

"What do you expect when you are expected to go from medium outdoor hard, to very fast, indoor low-bouncing court, to very slow outdoor clay court?

"This is exactly the reason why guys like Roger Federer and Rafael Nadal won't play the Davis Cup, even though they love the competition - because it's tough on the body to keep changing surfaces.

"So for me, the Davis Cup - although it was great winning and I loved playing in Birmingham - is very hard to fit into your schedule."

Murray moved to 10th in the world rankings on Monday after Tommy Haas failed to reach the US Clay Court Championships semi-finals.