The world number two took opponent Marcel Granollers and the Internazionali d'Italia crowd by surprise when he pulled out having just levelled the match at 4-6 7-6 (7/5).
It looked as if it was just precautionary ahead of the start of the year's second grand slam next Sunday but in his press conference Murray gave a downbeat assessment of his chances of playing in Paris.
He said: "I pulled out because there was a good chance I would be playing tomorrow [Thursday] if I got through. As it is, I'd be very surprised if I was playing in Paris."
It was certainly not the way Murray would have wished to celebrate his 26th birthday.
The US Open champion had only previously retired once mid-match during his career, which bizarrely also came on his birthday when he snapped a tendon in his wrist in Hamburg in 2007.
That injury is the only time Murray has been forced to miss an extended length of time, with the Scot sitting out the French Open and Wimbledon, and it is not yet clear whether he may need surgery or if a similar lay-off will be required.
Murray struggled with a lower back problem throughout the clay season last year in particular during the French Open, leading Virginia Wade to call him a "drama queen" when it was at its worst in a match against Jarkko Nieminen.
Murray had injections before Wimbledon that seemed to ease the problem but he revealed it got worse again in Madrid last week and his concerns were heightened by the fact it did not improve with rest.
Murray said: "It's exactly the same thing as last year. I need to make a plan as to what I do, speak with the guys tonight and the physio and come up with a plan for the next few days.
"I'll make a decision on Paris after the next five days I would have thought. I obviously need to take some days off and see how it settles down, but a few days didn't really make a huge difference this time so we'll have to wait and see.
"Until Madrid it had been okay but it's not been perfect for a long period. You always go into matches with little niggles and such, but it's frustrating when for a long period you're hurting.
"It's been an issue for a while now and I want to make sure it goes away. It's not enjoyable when you're playing in pain."
Murray revealed he has had pain in his back for the last 18 months, saying: "It's been here since the end of 2011 pretty much.
"But it got bad during the clay season last year. Obviously I had all the injections and that helped for a little while but it's been an issue for quite a long time."