And he appeared to be struggling with the illness during his first round-robin tie. Alcaraz seemed to be using vapour rub during his contest with Ruud and was spotted blowing his nose and touching his stomach.

The 21-year-old had chances to break in the first game. But he quickly let the match get away from him as Ruud pounced to lead 3-1.

The Norwegian saved more break-back points and continued his epic run, winning five games in a row to take the first set 6-1 after 36 minutes as Alcaraz looked helpless.

Alcaraz started to pick it up in the second set and got his first break, racing into a 5-2 lead.

But he still looked out-of-sorts and was broken while serving to force a decider. There were gasps from the crowd as Alcaraz sprayed errors around the court.

His signature drop shot abandoned him as he continued to dump balls into the net. And Ruud took advantage, breaking again to serve for the match.

Alcaraz buried his face in his towel as he headed to the changeover with the Norwegian back in the lead. The Spaniard managed to save the first two match points but he was only delaying the inevitable.

On his third match point, Ruud delivered a devastating blow with an ace to earn his first victory over Alcaraz in five meetings. He had previously only won one set in their last four matches.

It was also Ruud's first win since October 16. Since then, the world No. 7 has lost four matches in a row with opening-round exits in Basel, Paris and Metz.

Alcaraz finished the match with 25 winners to 26 unforced errors after struggling with some of his bread-and-butter shots. The world No. 3 was clearly under the weather and he now has two days to recover before his next match.

Alcaraz also lost his opening round-robin match last year but still made it out of the group and into the semi-final. But it will be more of a challenge to do the same this time if he isn't in full health.

The disappointing result comes after Marca reported Alcaraz had to train with a nasal strip under his physio's orders to help him breathe better while battling an illness. Even Ruud noticed his rival was clearly unwell during their clash.

"I kind of knew he was maybe dealing with a bit of a cold. I've seen him sniffling and always with a tissue for his nose. That's a sign that physically he won't be 100 per cent," he said.