PERTH, Australia -- ATP World No.1 Novak Djokovic is always keen for some mischief. The Serbian star was strolling by the interview room at the United Cup when he heard the familiar tones of Mandarin Chinese emanating from the speakers. 

So he did what he has done throughout his illustrious 20-year career. He decided to show off his language skills.

While China's Zheng Qinwen and Zhang Zhizhen basked in the glow of their victorious United Cup debut, Djokovic entered the room and delighted the players and reporters with a jaunty "Hello! Thank you all!" in Mandarin.

No one was as excited as Zheng Qinwen. The World No.15 gasped in delight and waved back. "Idol!", Zheng said as Djokovic left the room. With the 10-time Australian Open champion out of earshot, Zheng turned back and whispered into the microphone. 

"Please don't go, I still haven't got your signature yet," she said, before bursting into laughter. 

Watch the hilarious moment below: 

After their resounding 3-0 win over the Czech Republic on Saturday to move to 1-0 in Group E, China will face Djokovic's Serbian squad on Sunday. Zhang will take on Djokovic, while Zheng will face Olga Danilovic. 

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