Answer
Jürgen Melzer is the only person to have defeated Djokovic from two sets down, as of June 2023. This is an impressive feat, as Djokovic is one of the most dominant players in tennis history. Melzer defeated Djokovic in the second round of the 2010 Australian Open, coming back from two sets down to win 4-6, 4-6, 6-3, 6-3, 6-4.