The Amstel Gold Race is Haas' favourite event and one he targets each season in hopes of a breakthrough victory, however that may have to wait for another year as he starts the 2018 race with reduced expectations.
The 29-year-old has not felt well in training and on Monday will undergo medical tests in Belgium in hopes of revealing the problem. He will not race the mid-week Flèche Wallonne and Liège-Bastogne-Liège next weekend.
"It's no secret that this is my favourite race of the year, so it’s really hard to not be in my best condition. I’ve been trying to train through the illness, but unfortunately, my form is not where I would like it to be," Haas said.
The WorldTour race starts on Sunday 15 April in Maastricht and runs around the Limburg region for 262.6 kilometres.
"Even though it will be hard, I’m still super excited to be on the start line at Amstel," Haas said. "I’ll be looking at this year’s race as an opportunity to practice, and keep the race fresh in my mind, and hopefully be able to help some of my strong teammates get a result.
"Leadership isn’t only about racing to win; its also about sharing knowledge and experience, and this is how I’m approaching Amstel Gold this year."