Manchester City has conceded for the first and second times this preseason, and may give up their first points as well.
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West Ham almost went down early when Yaya Toure powered a header that Adrian pushed over the bar.
Sergio Aguero missed a chance to equalize when he moved around a sliding Adrian but pushed his shot through traffic wide of the frame from 17 yards.
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And Sakho netted off a scrum in the box to give the visitors a surprising 2-0 lead with an hour to play.
Adrian made a pair of tight saves on Aguero in the 40th minute, and sent the match to halftime 2-0.
Then De Bruyne calmly settled a pass from the left, setting up a curling low right-footed shot that beat a diving Adrian.