Roy went 9-of-13 from the field off the bench, hitting the go-ahead basket with 39 seconds remaining as the Trail Blazers overcame an 18-point fourth-quarter deficit to even their Western Conference quarterfinals series.
LaMarcus Aldridge put in 18 points and Gerald Wallace hauled down 11 rebounds to go with 10 points for Portland, which outscored the Mavericks 35-15 down the stretch to pull out the win.
Aldridge and Marcus Camby each blocked three shots and the Blazers held Dallas to 40.8 percent shooting.
Dirk Nowitzki scored 20 points for the Mavs, who will host Game 5 Monday.
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