The 'Gotta Be You' boyband are on the verge of becoming the first UK group to have their debut album top the Billboard chart in its first week of release with Up All Night.
Speaking to The Sun, the star confessed: "We thought it could be the same few hundred people tweeting us over and over again.
"Until we got to America, we didn't realise how crazy it was," the 19-year-old singer added.
They have already won favour with first single 'What Makes You Beautiful', which entered the Billboard Hot 100 at number 28.
Earlier this month, Malik took a brief hiatus from the American tour following the death of his aunt, returning later that week.
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Watch One Direction's US TV debut, singing 'What Makes You Beautiful' on Today below: