Former US Open champ
Robredo's surprise exit came after the Spaniard injured his left abductor muscle the day before during the second set of a fourth-round match against Sam Querrey.
Robredo said he only realised the extent of the damage after he got the results from an MRI scan doctors did on Wednesday. The injury will also prevent him from playing in next week's ATP tournament in Florida.
"I am out of here. And I am out of Miami, as well," Robredo said.
He said he sought advice from doctors in the Palm Springs area and in Spain about what to do.
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"We did an MRI yesterday night, and we saw the results," he said.
"Obviously the doctor here says not to play.
"We have been delaying it as much as possible because we were contacting our doctors in Spain to see what they feel and now the decision is done.
"Everyone is saying the same thing. I didn't even try. I knew I was not going to be able to play, so that's why I pulled out."
Robredo said he couldn't go into too much medical detail about the scan results but thinks he might have pulled two muscles.
"I think I strained the muscle in two places, so no chance to play," Robredo said.
"Well, a chance to play but then I wouldn't be running as normal and I would have had the chance to break it."
Del Potro, of Argentina, advances to the semi-finals where he will square off against either Rafael Nadal or Ivo Karlovic.
"I'm sorry for Robredo. I wish him a fast recovery," Del Potro said.
Del Potro has been one of the hottest players on the tour this year.
He take a superb 17-4 record into his first Indian Wells semi-final appearance which he hopes is against world No.1 Nadal.
"I am glad to be (in the) semi-finals. Maybe I could play with Nadal again," Del Potro said.
"We make very good matches before, so I have two days to be better, in good form, and hopefully I can play 100 per cent."
