The Slovakian just came from an altitude training block in Spain, where he had began to feel sick.
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The Bora-Hansgrohe team believe he picked up the virus while training at Sierra Nevada. Cycling Weekly learned he had diarrhoea for the last six days and looks as though he lost weight.
The three-time world champion, winner of the 2018 Paris-Roubaix, is one of the big-name starters of the Tirreno-Adriatico stage race along with Geraint Thomas (Sky). The Italian race runs almost parallel with Paris-Nice in France.
It is his third event of the season after starting the 2019 season with the Tour Down Under, winning stage three, and the Vuelta a San Juan. Sagan will be using Tirreno, where he has won seven stages in the past, to improve his form ahead of Milan-San Remo on March 23.
Despite the setback, the team is certain he will start the race on Wednesday in Lido di Camaiore. A pre-race press conference is scheduled for Tuesday afternoon, Sagan will not attend but was never on the list to do so regardless.
In his favour is a relatively short opening 21.5-kilometre team time trial on stage on that should allow him an extra day to recover. The team will aim for the overall with Rafał Majka, but brought a Classics team with Daniel Oss and Marcus Burghardt to prepare for the Classics run with Milan-San Remo, the Tour of Flanders and Paris-Roubaix.
Sagan is pushing all the way through to Liège-Bastogne-Liège this spring. With that in mind, he stayed in Sierra Nevada and skipped the Strade Bianche gravel race on Saturday. With the Classics run already starting next Saturday, both Sagan and the German WorldTour team will want him to participate at Tirreno to have a chance at winning Milan-San Remo for the first time.
In addition to the preparation, an inform-Sagan would have a chance to win almost every stage but the time trials.