While riding for Blanco Pro Cycling in 2013, Slagter won the Tour Down Under, his greatest career achievement. Slagter won the third stage after attacking on the finial climb, cresting it alone. He was joined by three riders on the descent, and with the peloton on their heels, Slagter out-sprinted the group. On the fifth stage, he gained the orange jersey; finishing atop Old Willunga Hill in second position, behind Australian Simon Gerrans (Orica–GreenEDGE), the defending champion. On the final stage, Slagter successfully defended his lead, with Spaniard Javier Moreno (Movistar Team) earning second place, seventeen seconds in arrears.
In August 2013, Slagter signed with the Garmin–Sharp squad, penning a two-year deal.