Murray, who turned 26 Wednesday, used a strong second-set comeback to pull even with Marcel Granollers at 3-6, 7-6 (7-5) but, after nearly 2 hours of play, was unable to come out for the third set and says he could miss the French Open.
Murray was treated courtside during the second set for an injury and fell behind 1-4 in the set before the comeback.
"I have an issue with my lower back," Murray said. "It's been an issue for a while. I would be very surprised if I was playing in Paris.
"I need to make a plan as to what I do. I'll chat with the guys tonight and make a plan for the next few days then make a decision on Paris after the next five days."
The French Open begins May 26.
Jerzy Janowicz ousted eighth-seeded Jo-Wilfried Tsonga 6-4, 7-6 (7-5), losing just 17 points in 11 games serving. Also Wednesday, Jeremy Chardy took out 16th-seeded Kei Nishikori 6-4, 6-1 without seeing a break point and Alexandr Dolgopolov was awarded a walkover when 15th-seeded Stanislas Wawrinka was unable to play because of a thigh injury.
The men's final match of the day saw fifth-seeded Rafale Nadal easily handle local favorite Fabio Fognini 6-1, 6-3 while No. 4-seeded David Ferrer got past Fernando Verdasco 5-7, 7-5, 6-3. If Nadal and Ferrer both win their next matches, the Spanish stars would meet in the quarterfinals, just as they did last week in Madrid.
First Nadal must play Ernests Gulbis, who was a 6-1, 6-1 winner over Viktor Troicki in the second round, and Ferrer must deal with Philipp Kohlschreiber, who advanced with a 6-4, 6-1 win over Albert Ramos.