On-the-whistle report: Rotherham United 0 Rochdale 1
The Millers had been in a play-off spot thanks to their superb results on the road but their stutters at AESSEAL New York Stadium continued as a Dale side who had lost their previous five league matches beat them 1-0.
A stunning 42nd-minute strike from Aaron Morley was the difference between the two sides on an afternoon when Paul Warne's men slipped four places in the table to eighth.
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Hide AdRotherham have won only three times in the league on their own turf this season and Dale have been victorious on every visit they have ever made to New York.
The Millers dominated the second half but could find no way past spirited visitors.
The first period was a lively affair despite there being few clear-cut openings.
Rotherham were close to an early opener when Matt Crooks headed against the post after eight minutes and Ben Wiles' follow-up shot brought a good save from Robert Sanchez.
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Hide AdMost of the Millers' danger was coming down the right flank from Chiedozie Ogbene and his 38th-minute cross found Carlton Morris alone at the back post.
Morris got plenty of power on his header but little accuracy and the ball flew over the bar.
The home side found themselves behind three minutes before the break when Callum Camps' dummy created the chance for Morley to sweep home a superb effort from the edge of the penalty area.
Rotherham piled on the pressure in the second but found a breakthrough impossible to come by.
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Hide AdCrooks side-footed wide when he should have hit the target from Ogbene's low cross, while substitute Jake Hastie's supply from the left found Ogbene who headed straight at Sanchez.
Hastie went alone just after the hour mark, jinking inside and shooting into Sanchez's hands, then Ogbene tumbled inside the box but loud appeals for a penalty were in vain.
With 70 minutes gone, Matt Olosunde's cross set up Clark Robertson whose low shot was gratefully gathered by Sanchez.
Dale threatened for the first time since the interval four minutes later as Matt Done flashed a shot just wide while Camps' long-range free-kick on 84 minutes brought a stop from Daniel Iversen.
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Hide AdThe Millers pressed again but Dale held out and there were some boos at the final whistle.
Rotherham (4-4-1-1): Daniel Iversen; Matt Olosunde (Kyle Vassell 73), Michael Ihiekwe, Clark Robertson, Joe Mattock; Chiedozie Ogbene, Ben Wiles, Jamie Lindsay (Michael Smith H-T), Carlton Morris (Jake Hastie 60); Matt Crooks; Freddie Ladapo. Subs not used: Lewis Price, Adam Thompson, Trevor Clarke, Dan Barlaser.
Rochdale (4-2-1-3): Robert Sanchez; Luke Matheson, Eoghan O'Connell, Rhy Norrington-Davies, Jimmy Keohane; Jimmy Ryan (Matthew Gillam 66), Aaron Morley; Callum Camps; Matt Done, Ian Henderson (Fabio Tavares 90+2), Rekeil Pyke (Kwadwo Baah 77). Subs not used: Jay Lynch, Lewis Bradley, Harrison Hopper, Ethan Brierly.
Goals: Morley 42 (Rochdale).
Referee: Chris Sarginson (Staffordshire).
Attendance: 8,313 (262).