The visitors appeared destined to claim all three points in Tuesday's League One clash at Bramall Lane when Kevan Hurst and Will Grigg netted quickfire efforts early in the second-half.
But goals from Matthew Lowton, substitute Lee Williamson and a Richard Cresswell penalty ensured United preserved their 100 per cent start to the new season.
After a goalless first half, Hurst broke the deadlock when his shot deceived Steve Simonsen almost immediately after the restart before Grigg doubled Walsall's advantage when United failed to clear a corner.
Lowton pulled one back following some fine work by substitute Lee Williamson and Dean Smith's men simply crumbled.
Williamson levelled after combining well with Lecsinel Jean-Francois before Cresswell, who moments earlier had seen a header cleared off the line, sealed the points after Darryl Westlake had felled the former Rotherham midfielder in the box.
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