Kevin Durant was down in the dumps — again — through the first half, frustrated for the third straight game and, this time, virtually invisible in the third quarter. The best scorer on the planet had attempted as many shots going into the final period as Mavs role players Mike James and Jae Crowder. Then it clicked. The light went on and Durant went off. Durant scored 19 of his 31 points in the fourth quarter to lead the Thunder to a 107-101 victory over Dallas on Sunday night inside American Airlines Center. Durant played the entire fourth quarter, made seven of 11 shots, two of three 3-pointers and all three of his foul shots. “I thought at some point in the game he kind of got frustrated a little bit,” said Thabo Sefolosha, who stuck the dagger in Dallas on a fadeaway with 12.5 seconds remaining. “It was good that he was able to turn it around and put it on himself to just work harder and get some baskets. That was great on him. It shows maturity on his part.” After sitting on 12 points on 3-for-5 shooting through halftime Sunday, Durant has now averaged just 10.7 points on 40 percent shooting in the past three first halves. In the third quarter against the Mavs, Durant was scoreless in eight minutes, missing all three of his shots while grabbing just one rebound and turning it over once. He finished the game with just one assist against six turnovers.