Best way..

nltrainer
Traveller
1404 comments

Not possible-ALL (well, nearly all|) cross-border trains into Serbije have been cancelled since some +10 yrs now.
In fact -unless you are married with trains=go out of your way to ride in them instead of other means-this is a near impossible trip as ALSO GR has NO cross border trains anymore.
best site for a general advice: seat6.com. Lists the trips from UK, so from here (Nederland, I too sit here) you can use it also for most of it.
Best way on a pass is since that time via Italy and the ferry to GR. Note there are (or at least were) many MORE ferries as those shown on the IR-planner-these may be even cheaper too, without a discount.
Going that way on a pass is in fact only worthwhile if you also plan to make some stops along the way you choose.
Mr google has apparently sent you to this site, FAR better-and your Q is there now a few million times: community.eurail.com. As since some 4-5 yrs Eu and INterrail work about the same (not before)

MisterSteve
Traveller
1089 comments

There are no international train connections into Greece and no info on when they will resume, since Serbia is in such a mess the Bulgarian route would probably be the first. Until then it is possible to get close to Greece by going to Sofia and the getting a bus from there (not included in Interrail). Theoretically Greek trains are running to Thessaloniki but their idea of what makes a good service (and what makes good information) may not meet yours so a bus all the way from Sofia to Athens might be better. https://www.hellenictrain.gr/en/intercity-network https://www.busexpress.bg/en/destination/sofia/athens