Alcaraz has now raised alarm bells for the rest of the tournament, admitting it was tough to see himself getting out of the group stage and into the semi-final.

The Spaniard still has two more matches to play on Wednesday and Friday. If he wins them, he could still have a good chance of reaching the last four.

But that seems out of reach for last year's semi-finalist, who confirmed he got sick ahead of the tournament and struggled during his 6-1 7-5 defeat to Ruud.

"Well, a few days before coming here, I got sick at home," Alcaraz said after the match. "And yeah, the days that I was practicing here, I was feeling okay.

"Not pretty good, but okay. I could play. I could feel that I can get into the rallies in the practice. Obviously, in the matches is totally different."

Alcaraz seemed to be battling with a cold in the days before the tournament. And he revealed that his stomach started to bother him on Monday.

He continued: "But today I didn't feel well. In this morning, I feel uncomfortable in the stomach. After long rallies today, I couldn't feel well.

"I don't want to, you know, say because I don't want to sound like an excuse. But if I feel bad, I feel bad. It is what happened today."

The four-time Grand Slam champion is now doubtful that he can make it beyond his round-robin matches. Alcaraz has been drawn in the John Newcombe group with Ruud, Alexander Zverev and Andrey Rublev.

He still has to play the latter two men later this week. Last year, Alcaraz also lost his opening match but won the next two to make the semis. However, he can't see the same happening this time.

"Based on the feelings I had today, it is difficult to think about the semi-finals," he told Spanish media.

"The good thing about tennis is that you can be very bad one day and improve a lot for the next match. I myself did not have good feelings last year in my debut and I was able to qualify for the semi-finals. I want to feel better as the days go by."