John Isner said it had been "very, very difficult" to focus on his second match at the ATP Finals against Marin Cilic after learning of the death of his close friend and former strength coach Kyle Morgan.
Morgan, a 35-year-old former Florida University football player, was found dead on Monday after being reported missing by his family.
"Found out two nights ago after I lost to Novak (Djokovic)," the 33-year-old said after his 6-7(2) 6-3 6-4 defeat -- his second loss on his debut in the tournament. "It's difficult, very difficult. Got to go out there and play.
"When I left college, I worked out exclusively with Kyle for probably seven years. That's four, five times a week for seven years. We lived together, as well.
"Certainly very close. It's devastating not just for me, but for a whole bunch of people. He was a fantastic guy."
Earlier the world number 10 had posted a message on his Instagram page, alongside a photo of the two of them.
"It's hard to explain how much I loved you and how instrumental you were in making me who I am today. My heart and everyone who knew you is absolutely broken," Isner wrote.
"You touched so many lives in the best possible way.
"Despite our crazy differences, we forged an inseparable bond. You were the most knowledgeable strength coach ever and the years we spent in the gym together allowed me to compete at the highest level.
"Kyle Steven Morgan, I love you bro. Gone but never forgotten."
Isner will face Germany's Alexander Zverev on Friday still with an outside chance of reaching the semi-finals.
