He was skinnier back then. He also wore Seattle SuperSonics green. Kevin Durant was a wispy 19-year-old rookie the first time Spurs guard Manu Ginobili laid eyes on him, not yet a household name after one college season at Texas and nowhere close to the player he would become. Still, it didn’t take much for Ginobili to forecast a frightening future for the rest of the Western Conference. “You could tell he was going to be a star,” Ginobili said. Tonight, Durant returns to the AT&T Center with the Oklahoma City Thunder, long since relocated from the Pacific Northwest, the embodiment of the Spurs’ worst fears. Having helped jettison the Spurs in last season’s Western Conference finals, the 24-year-old Durant — in search of a nearly unprecedented fourth straight scoring title — has grown into the biggest obstacle in the Spurs’ quest for a fifth NBA Finals berth. With a win tonight in the latest installment of this budding rivalry, the Thunder (47-16) would match the Spurs (48-15) atop the West standings for the first time since before the All-Star break. “We want to continue playing our best basketball and get back to where we were a couple of weeks ago before the break,” Spurs guard Danny Green said. “All these games are great tests to see where we’re at and where we want to be come playoff time. (OKC) is probably the ultimate test when it comes to that.” Coming off the most lopsided loss in AT&T Center history — a shocking 136-106 humiliation against Portland on Friday — the Spurs continue to insist personal improvement trumps playoff seeding at this point of the season. Sunday’s practice and film sessions came with a heavy emphasis on defense, after a game in which the Spurs allowed the Trail Blazers to shoot 60.2 percent, including 13 of 21 from 3-point range, and padded Damian Lillard’s Rookie of the Year campaign to the tune of 35 points.