The Wildcats (3-1) surrendered a 15-point lead with 3:05 to play in the third and were outscored in the fourth, 21-10, as the Cowboys (3-0) drilled four of five 3-pointers during that stretch.
"We had a lead most of the game and the kids got complacent," Renaissance Coach Sid Cooke said. "We just kind of closed out mentally a little bit in the end. It cost us, but it's good that we got it out of the way before the real season starts."
Renaissance power forward Jessy Cantinol led all scorers with 18 points and six rebounds. The senior poured in eight of his 10 halftime points in the first quarter.
Vince DeGuzman, a senior shooting guard, added 13 points, including three three-pointers.
The Cowboys' Clay Kasdorf (15 points) hit a driving layup and drew the foul. He failed to complete the three-point play, preserving the Wildcats' lead at 46-45 with 2:29 to play.
De Guzman buried a pair of foul shots the next time down the court, but Alex Vaughn sank a 3-pointer from the corner to tie it at 48 with 2:05 left.
Gerrick Uneau (six points, nine rebounds) picked off a steal and tipped it ahead for a Cantinol layup.
But the Cowboys struck back quickly, with Coley Apsay (team-leading 16 points) draining a three-pointer for the first Canyon Country lead since it was 5-2 early in the game.
Trailing, 51-50, with 28 seconds left, Cantinol missed a turnaround shot in the paint, but recovered it before he got fouled.
Cantinol missed both free throws and the Wildcats were forced to commit a foul with five seconds left. Kasdorf hit both from the line for the final margin and De Guzman's 30-foot attempt was tipped as time expired.
"First three quarters, we executed our plays and listened to plays. Fourth quarter, we got away from what the coaches were telling us, bad turnovers, just bad mistakes," De Guzman said. "It was just dumb mistakes. That's all it was. We threw the game away ourselves. It's a learning experience. We learn from this, we'll get better from it."
The Wildcats have not won the tournament since 2007, when Cooke took over as coach.