I think it depends on how noisy the stage is. I heard a pair of SM81's was used for Ravi Shankar's concert.
If the venue is noisy you can try a handheld condenser vocal microphone. I had good results with a Beta 87A placed as close as possible and pointed at 3-4 cm above the bridge.
The proximity effect isn't a big problem if the sitar player belongs to Imdad khani/Senia Gharana (Vilayat Khan etc.) as the Ga string is used instead of the low Sa. If the player belongs to Maihar Gharana (Ravi Shankar etc), then his sitar has the low Sa string and he probably uses a double-toomba sitar (which has more low) so the proximity effect can be a little problematic.
Make sure you bring a pre with plenty of gain. Edit: I now realized this thread was posted more than three years ago. sorry! 