(CNN) Rafa Nadal criticized fans for showing a lack of respect at the ATP Cup following Spain's defeat by Serbia in Sydney.

The world No. 1 fell 6-2 7-6 to Novak Djokovic as Serbia clinched the inaugural team event 2-1.

The Spaniard appeared to be irked when he trailed 5-2 in the first set, speaking with the match umpire before giving a sarcastic thumbs up to a noisy section of the crowd.

"Honestly the crowd was fantastic every single day but sometimes people from some countries, they probably don't understand how tennis goes," Nadal told reporters after the loss.

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