Novak Djokovic admitted his season had come to an "awkward end" after Roger Federer withdrew from Sunday's ATP World Tour final in London.
Federer, 33, pulled out with a back problem after a lengthy semi-final against Stan Wawrinka on Saturday.
That left world number one Djokovic to collect the trophy before playing an exhibition set against Andy Murray.
"You cannot blame Roger. I'm sure he would play the World Tour finals if he could," said Djokovic, 27.
"This is probably the first time in history that this happens. It's a very awkward situation to talk about, to be honest.
"You never like to win, especially these big matches against big rivals, with the retirement, but that's the way it is."
Djokovic only learned that Federer was withdrawing during the preceding doubles final, and the Serb paid tribute to Murray for stepping in as cover.
ATP executive chairman Chris Kermode said he called Murray at 14:00 GMT, and the Scot "instantly" agreed to travel to the O2 to play a set of singles, followed by doubles with John McEnroe, Tim Henman and Pat Cash.