Chris Horner (Lampre-Merida) abandoned Tirreno-Adriatico before Monday’s stage 6 because of tendinitis in his left Achilles tendon.
The American was 11th in the GC after stage 5, 3:06 behind leader Alberto Contador (Tinkoff-Saxo).
“Horner is suffering from tendinitis at the left Achilles tendon,” team doctor Beltemacchi Matteo said in a press release. “As jointly agreed, in order to avoid a worsening of the situation, Chris will quit the race.”
Lampre’s medical staff will evaluate Horner in the coming days and put together the best course of treatment for the 42-year-old.
“Leaving T/A with a sore Achilles. First time here in 1998 four more times after that, and I have never left in one piece,” Horner wrote in a series of Tweets. “But! I have been 2nd and 6th on GC. Wore the leaders jersey for many days. And top ten in stages more time then I can remember.
“Got to take the good with the bad sometimes!”
The setback comes after Horner began the 2014 season unemployed, despite winning the Vuelta a Espana last September. Lampre eventually signed him to a contract in January.
Horner finished eighth in the Vuelta ao Algarve last month. He is expected to be Lampre’s GC leader at the Giro d’Italia in May.