The world No2 was addressing the Spanish media after his impressive 7-6, 6-1, 7-5 win over the former Flushing Meadows semi-finalist when he grimaced in pain, slumped in his chair and slid to the floor out of view clutching his right leg.
Medical staff were summoned and after 15 minutes of treatment inside the Arthur Ashe Stadium, the 10 time grand slam winner returned to his feet and insisted he had merely been suffering from cramp.
“It was just cramping in the right leg, in the front and in the back. It was very painful, that's all,” said Nadal, who resumed his news conference standing instead of sitting.
He insisted the incident will not affect his preparations for his fourth round match against Luxembourg’s Gilles Muller, scheduled for tomorrow.
“I will train normally [on Monday]. It was just a normal cramp that could have happened anywhere, but it happened in the press room. Anywhere else, nobody would have noticed.”
During victory over Nalbandian, played out in 84 degree-heat, Nadal had also required treatment on his blistered right foot.
Elsewhere, Spanish fifth seed David Ferrer defeated German 26th seed Florian Mayer 6-1, 6-2, 7-6 and next takes on 2003 winner and home favourite Andy Roddick.
The American 21st seed, playing in his 12th successive US Open, brushed aside French wildcard Julien Benneteau 6-1, 6-4, 7-6.
Fifth seed Juan Martin Del Potro, a winner here in 2009, however, was beaten by Gilles Simon in four sets.