Drizzle shortly after the start made for an intense race in Baltimore albeit one that was spoiled by rather too many caution periods.
Nonetheless it set up an intriguing contest for the championship in the final race at Fontana.
Power led at the start but as the rain began to fall he and several other drivers made for the pits and switched to rain tyres.
Hunter-Reay had already picked up a few places and gambled on staying out on slicks. It paid off and he was able to make his first pit stop to refuel while Power was struggling on worn wet tyres on a drying track.
Hunter-Reay picked his way back through the field, eventually taking second behind Sato. He then passed the RLL driver after one of the race’s eight caution periods.
His final pit stop dropped him back into the pack with Power temporarily in the lead before he too had to pit. Briscoe inherited the lead but was running low on fuel. Another caution period saved him but Hunter-Reay snatched the lead with a rapid reaction to the green flag which left the Pesnke pitwall seething.
Hunter-Reay nearly outbraked himself at the first corner but scrambled around to hold Pagenaud off. But there was to be yet another restart as a multi-car crash ended up with one car parked on top of another.
The final restart came with two laps to go. Power, now fifth, came under attack from Barrichello at the hairpin and lost another place to Servia. He was able to take one place back before Hunter-Reay took the chequered flag for his fourth win of the season.
Briscoe, second, ran out of fuel after crossing the line. Pagenaud was third ahead of Dixon, Barrichello and Power.
The result puts Hunter-Reay firmly back into contention with Power ahead of the final race of the season: the 500-mile race at the Auto Club Superspeedway a week on Saturday.