The Hellas Express night train runs from Belgrade in Serbia to Thessaloniki in Greece. The full journey, via Skopje in Macedonia, will take you about 15.5 hours.
Although this is one of the fastest ways to travel to Greece, keep in mind that Macedonia isn't included in any of the Eurail passes. This means that you will have to pay an additional fee for this night train when traveling through Macedonia.
Update June 7 2016
The Hellas Express night train travels from June 20 to September 3. Use the Eurail timetable to find alternative trains and routes.