Shanghai - Olympic hurdles champion Aries Merritt revealed on Friday that he needed a second operation for kidney trouble and said he was "months" behind in his preparations for the Rio Games.
The 30-year-old American underwent a kidney transplant in September but he said he then needed a second procedure after complications developed.
The latest details about Merritt's condition raise questions about his ability to mount a successful Olympic title defence. He said he still suffers discomfort when he trains and competes.
"About seven weeks later, I actually had a second surgery that many people don't know about," Merritt told journalists ahead of the Shanghai Diamond League.
"I had one complication which caused me to have to go back in and have another operation... I was having difficulty with haematoma that had developed that was actually crushing the kidney."
Merritt was back training in by January but he said his preparations were way behind those of his rivals. He finished sixth in the 110 metres hurdles at this month's Doha Diamond League.
"It is a struggle, but it's not because over time it will get better and better. But I don't want to make any excuses because I'm lining up with everyone else," Merritt said.
"It's like I'm months and months behind my competitors (in terms of training), but I'm still running decent," he said.
"I'm really strong right now but I'm just not fast," Merritt added with a laugh.
Merritt said his blood chemistry is now normal following the transplant -- using his sister's kidney -- which was necessary because of a genetic disorder.
"All my labs are great. My basic body chemistry is normal. My blood levels are normal," he said. "Now, I'm full of energy and I'm ready to try to get back to where I was before."
In Shanghai, he will face fellow American David Oliver, who won the world title in 2013, and Jamaican Omar McLeod, who won the world indoor gold in Portland earlier this year.