Kei Nishikori is questionable for the upcoming Masters tournaments at Indian Wells and Miami after retiring with a left hip flexor injury at Delray Beach.

Nishikori was coming off a title run at the U.S. National Indoor Tennis Championships in Memphis. Nishikori said his hip began to bother him prior to the tournament and had nothing to do with a minor traffic mishap on Wednesday, when the car he was riding in was rear-ended.

"It was unfortunate that happened. But it didn't make worse my injury," said Nishikori, who plans to have an MRI and said he is uncertain about playing Indian Wells and Miami.