Watch! Novak Djokovic challenges fan during tense Australian Open match against Alexei Popyrin

Defending champion Novak Djokovic challenged a heckler to “say that to my face” during the 4th set of a testy second-round win over Alexei Popyrin at the Australian Open on Wednesday.

Before the start of the fifth game of the fourth set, Djokovic walked back behind the baseline and engaged with a spectator, gesturing to him to come down and “say that to his face.”

That match had been going for almost three hours by then, and Djokovic had dropped the second set and had to save set points in the third.

After the outburst, he won three straight games to take the match away from the Aussie player en route to a 6-3, 4-6, 7-6 (4) 6-3 victory.

After clinching the match on a Popyrin error, he turned around to the crowd again and yelled, pumping his fist to celebrate.

Djokovic has been dealing with a sore wrist and said after his opening match — a four-hour, four-set win over 18-year-old qualifier Dino Prizmic — that he hasn't been feeling well.

He credited Popyrin with bringing out a game plan to unsettle him.

“I haven’t been playing my best, but still trying to find form,” the 36-year-old Djokovic said. "Particularly in the early rounds, you play players that have nothing to lose. They come out on center court to play their best tennis.

“Hopefully I’ll be able to build this as the tournament progresses.”