Onward Tickets from Belgrade to Istanbul

Do I need an onward ticket to enter Serbia?

If there is reasonable doubt that he/she will use the intended visit for purposes other than those declared.

Border authorities may ask to produce for inspection a tourist travel voucher issued by a travel operator (notarized), a duly certified invitation letter from a natural person or a legal entity, a return flight ticket, a proof of paid hotel booking or a certificate verifying the private residence address during his/her visit, a proof of paid health insurance as well as a health insurance policy for the period of stay in the Republic of Serbia covering possible medical costs that cannot be less than EUR 20,000.