The young Aussie made a perfect start to his one-off series start, with a comfortable lights-to-flag win in the 20-minute opener.
His cause was given a big boost when Luca Giacomin, who had pipped him for pole and ran second in the early stages, spun at the first corner.
From there Piastri was able to cruise to victory, his final margin over Nathan Herne more than three seconds.
TCR Australia regular Dylan O'Keeffe finished a somewhat lonely third, more than eight seconds behind the winner, with Thomas Randle taking fourth.
Points leader Haber endured a difficult first race, a clash with Ash Sutton leaving him in a lowly 14th at the finish.
The upside was that he started the reverse-Top 20 race from the third row, setting him up for a dominant win. The V8 Touring Car regular grabbed the lead on the opening lap, after which he was untroubled on his way to an eight-second victory.
Randle and Piastri played starring roles in the second hit-out, charging from the back of the Top 20 to round out the podium.
Piastri almost event made it all the way to second, he and Randle separated by less than a second at the end.
George Miedecke and Brett Holdsworth rounded out the first five, while Herne could only make it up to 12th.