Madrid, April 28 -- Tennis players are facing an unknown opponent at the Madrid Open. A stomach virus or food poisoning has affected 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 rebounded from a set and a break down against Osaka in their fourth round contest. She prevailed against No. 15 Osaka 6-7 (1), 6-3, 6-2 to reach the quarterfinals and stay on track to defend her title. "Oh my God, that was an incredible level," Sabalenka said. "She played incredible tennis. I feel like I just got lucky in a couple of shots in the third set, that's why it went that fast." Sabalenka won her 15th straight match and advanced to her 17th straight quarter-final. Sabalenka said she was spending as little time as possible on site. Gauff vomited on the court on her way to a victory over Sorana Cirstea on Sunday. Cilic couldn't play his match against Joao Fonseca on Friday. Gauff was eliminated in a three-set battle with Linda Noskova decided on a final set tie-break. World number three Gauff, who had been dealing with a stomach bug, fell to a 6-4, 1-6, 7-6 (5) defeat by Noskova. In the men's third round, Daniil Medvedev, who has reached the quarter-finals in each of the last two years, beat Nicolai Budkov Kjaer 6-3, 6-2....