Nadal was an overwhelming favorite in Australia to win his 14th Grand Slam title, but he picked up the back strain warming up and was never the same.
Nadal defeated del Potro 4-6, 6-3, 6-4, in the in the BNP Paribas Open final on Sunday.
Rafael Nadal overcame a slow start to beat Stanislas Wawrinka 6-4, 6-1 in the quarterfinals of the Rome Masters.
Dina Safina white-washed Britain's Anne Keothavong 6-0, 6-0.
Nadal was perhaps short of recovery time after semi-final win over Moya.
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