Al Hilal slump in semi-final first leg
Midfielder Ghasem Hadadifar's 56th-minute goal gave the Iranians victory at the Foolad Shahr Stadium in Isfahan ahead of the return leg in a fortnight's time.
Swedish midfielder Christian Wilhelmsson wasted a glorious opportunity to grab an away goal for Al Hilal when he failed to convert a 42nd-minute penalty.
The result meant Al Hilal, coached by former Olympique Marseille boss Eric Gerets, remain without an away win in the continent's premier club competition this season.
The Saudis were missing a number of first-team players but their seven-year unbeaten home record in the Champions League will give them confidence they can progress to the final in Tokyo next month.
In Tuesday's far more entertaining semi-final Saudi Arabian side Al Shabab came from behind three times at home to beat South Korean's Seongnam Ilhwa 4-3.