I can't think of any examples to be honest. Nadal had Wimbledon 2018 semifinal, Federer had Wimbledon 2019. These are more than just heartbreaking losses, these are matches which they lost only because of lack of mental strength. But I can't find any examples of Djokovic losing a match where he was the better player and "should" have won. The fact that some of his fans are crying about RG 2013 (a match where he was outplayed completely. No normal tennis fan will ever claim he was the better player in this match) just proves that he didn't really have heartbreaking losses in his career. But still, what was the MOST heartbreaking?