I am disappointed but its no where near as bad as his FO13 lose to nadal. USO 2013 deserves a mention as well. FO15 wawrinka was too good and novak wasn't even close to win, so its not such a big deal.
I'm not a nolefan, but I think 2013 was worse as defeating peak Rafa at RG is the hardest single challenge tennis has ever had. It'd been some bragging rights. Wawrinka played god tennis, nothing he could've done there
In FO15, Novak had to contend w/ a weather deIay in SF vs Murray so that aIone set him on the wrong foot in the finaI. This has got to b one of the biggest setbacks considering the various records on the line along w/ a relatively weak & inexperienced draw to the final. Added to the DQ fiasco is absence of WB this season making it like a double whammy so he’ll have to forget about HC season & bring his best to his least successful major @ RG.
For me it is USO'20 a bit more, because after RG'15 semifinals It was obvious that Djokovic would be more tired than Stan come sets 4 & 5 on the physically most demanding GS surface and that he could wait on another chance for CGS, and here Djokovic was a clear favourite for the finals & title after Tsitsi_passed away vs Coric.
Even in the case of the sixth possible loss in USO final, it still would've been another GS final and the most USO finals in The Open Era.