Sebastian Szymanski fired Rennes ahead against PSG with a blistering left-footed drive from the edge of the box.
Following their European Super Cup triumph, PSG stumbled in the French Super Cup and have now fallen behind early in their Ligue 1 curtain-raiser.
Squaring off against Stade Rennais, it was the hosts who drew first blood. Midway through the first half, a clinical counter-attack saw Sebastian Szymański collect the ball on the edge of the area.
He unleashed a venomous left-footed strike that caught Safonov completely off guard, leaving the goalkeeper helpless as his desperate dive came too late to stop the effort.