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The pressure of expectation now seems to weigh heavy on the 22-year-old's shoulders. His self-belief at a low ebb, Djokovic showed little aggression against a confident Haas.

"It's unbelievable. I have no words for it. I am playing some of my best tennis right now," said an overjoyed Haas after recording the biggest shock of the men's tournament so far.

The German always looked in control in the first set. He held his serve to love three times and broke Djokovic in the 11th game to ratchet up the pressure at just the right time.

In an engrossing second set tiebreak, Djokovic had three set points to draw level but Haas fought back from 6-3 down to take a comfortable lead.

"That was huge and maybe the key to success today. I said to myself - Wake Up," said Haas, a former world number two whose career has been blighted by injury.