Diouf, who arrived at the Britannia Stadium as a free agent in June following a successful spell with Hannover, popped up with an equaliser just before half-time at the Mage Solar Stadion.
Mark Hughes' men conceded an early goal with some uncharacteristicly poor set-piece defending, but recovered to claim a share of the spoils seven days before their Premier League opener against Aston Villa.
Pavel Krmas found himself completely unmarked in the third minute to head home a corner for Bundesliga side Freiburg, giving Stoke goalkeeper Asmir Begovic no chance.
At the end of a first half of few chances, former Manchester United forward Diouf drew Stoke level, latching onto Ryan Shawcross' long ball and showing good strength to hold off his marker before coolly slotting home.
Stoke started the second half strongly and Glenn Whelan's powerful shot from the edge of the area after a neat passing move was well saved by Freiburg goalkeeper Roman Burki.
Substitute Charlie Adam's crisp left-footed shot could have put the Premier League side ahead in the 72nd minute, but Burki again denied Stoke.
The Bundesliga side nearly took the lead themselves late on when a low cross from the right deflected off Robert Huth to beat Begovic, but the ball did not have enough pace to find the net and was cleared by Shawcross.
Stoke also had a chance to win the game in the closing stages, but substitute Peter Crouch's late diving header was straight at the impressive Burki.
