New Delhi, April 28 -- Tennis players are facing an unknown opponent at the Madrid Open.

A stomach virus or food poisoning has affected Iga Swiatek, Coco Gauff, Marin Cilic and a few others, causing some concern.

World No. 1 Aryna Sabalenka avoided an upset by Naomi Osaka on the court on Monday and said she's trying to avoid illness by sticking to a simple diet of chicken breast, rice and salad.

The rumor among the players was bad shrimp tacos were to blame.

Sabalenka knocked on wood and said, "So far, so good. I heard that I have to avoid those tacos (laughing). I stick to the same food, same meal that I've been having since the very beginning of the tournament."

Sabalenka said she was spending as little time as possible on site at ...