By Jan Strupczewski BRUSSELS (Reuters) - Greece has already received debt relief from its euro zone creditors equal to half of its 2013 GDP in net present value terms, the euro zone bailout fund said in its annual report, noting Greek public debt was now high but sustainable. Greece has made debt relief a key demand in its negotiations with international creditors on what reforms it must implement to secure further funding. The report from the European Stability Mechanism (ESM) said that after various modifications made to the original terms of bailout loans over the last years, Athens had minimal payment obligations until 2023.