Jack Sock has revealed that he was forced out of his room at 4am on Tuesday morning due to a fire alarm at his hotel.
The incident came 10 hours before Sock was due to take to the court against Marin Cilic, but the American showed few ill effects as he outlasted the Croatian at London's O2 Arena.
Sock ran out a 5-7 6-2 7-6 winner to keep his hopes alive at the end-of-season tournament, but the debutant acknowledged that he had experienced an "interesting" day.
The 25-year-old is quoted by BBC Sport as saying: "That was a tough one for sure. It's been an interesting morning so far, the fire alarm went off at 4am and we had to exit the building.
"But I love playing here in London, it's an amazing atmosphere, you make me feel like home. I'm just excited to win and keep myself alive."
A win over Alexander Zverev in the final group match on Thursday could see Sock progress through to the semi-finals.