Japan's Yoshihito Nishioka retired in his fourth round match at the BNP Paribas Open on Wednesday due to back pain.
Nishioka looked sluggish from the outset and dropped the first set 6-4 to Serbia's Miomir Kecmanovic despite taking medication and a time out to receive a massage from a trainer. He pulled out partway through the first game of the second set.
The 23-year-old Japanese had booked his round of 16 match with the Serbian, who reached the main draw as a lucky loser, after rallying past Canada's Felix Auger-Aliassime 6-7(2), 6-4, 7-6(5) on Monday.
"I wasn't worn out or anything (by the previous match), but I suddenly felt pain in the middle of my back during the warmup right before the match," Nishioka said.
"Serving was particularly hard. I was unlucky today."
Nishioka, who was hoping to improve on his fourth-round result in Indian Wells two years ago, said he will skip next week's Miami Open and return to Japan in order to undergo a physical.
Despite the unfortunate result, Nishioka praised his 19-year-old opponent, who is enrolled in the same tennis program at Florida's IMG Academy where Nishioka trained until he turned professional in 2014.
"The last time we practiced together was two years ago. He's getting stronger," said Nishioka, who earned his first ATP title at last year's Shenzhen Open.