Carlos Alcaraz revealed that he struggled with illness during his opening game loss at the ATP Finals to Casper Ruud on Monday.
Alcaraz looked out of sorts as he fell to a 1-6 5-7 defeat to Ruud in just 86 minutes in Turin, and now faces an uphill struggle if he is to progress from the John Newcombe Group.
During the contest, the Spaniard hit 26 unforced errors compared to his opponents' 10, while also failing to save any of the four break points he faced.
It also marked Alcaraz's first loss against Ruud in their fifth match against each other, with the 21-year-old confirming he had felt unwell leading up to the tournament.
“I don’t like talking about things like this because it sounds like an excuse and I don’t want to take anything away from what Casper did to win the match,” began Alcaraz.
“Before coming here, I was unwell at home. When I got to Turin, practice was going well; I could handle long rallies, competing and playing.
"And I hoped things would get better as the days went by.
"I don’t know if it was because of the nerves of the first match, but I didn’t feel completely well. My stomach limited my game a lot and made me feel uncomfortable.
"I could’ve done better. I’m not going to feel good in every match I play throughout the year, but it really limited me in terms of my recovery.”
Alcaraz will be back on court on Wednesday, when he takes on the loser of Monday's clash between Alexander Zverev and Andrey Rublev.
Failing to win would see the four-time grand slam champion fall short once again at the year-end tournament, but he was confident of claiming a maiden ATP Finals crown.
“I don’t want to think much beyond today... Given how I’ve felt, it’s difficult to think about the semis,” said Alcaraz.
“The good thing in this sport is that your game can feel very bad one day and very good the next.
"There have been many players who have lost the first match of the [Nitto] ATP Finals and ended up winning. I reached the semis last year after losing on the first day.
"Now I want to take it day by day, to feel better, and see how the next match is. I’ll approach it as best I can. Let’s hope things turn out well.”