Do you have an opponent against whom you fare far worse than you should based on ratings alone? If so, you're not alone. In my World Chess column this week I discuss the phenomenon of the difficult opponent, recounting several top-level examples with a focus on the battles between former World Champion Tigran Petrosian and Lajos Portisch. Finally, the piece concludes with some tips on how to deal with such opponents. Hopefully they will be helpful, and if you have some suggestions of your own please pass them along (especially to the combox there).