I think serena is powerful with her serves. Deals out disasterous aces when she needs to but I also think Justin is capable of beating her.
On the question of fear, i think as a world class player she must have dealt with the issue of fear a long time ago and replaced it with self confidence. So i dont think Justin is scared of serena. In any event it could be serena who is scared.
On a question of fear everyone is scared of Serena, you can ask Sharapova about that. it is well known that Justine is scared of Serena. As a champion i think right now Justine is second best to Serena even though she has the number 1 ranking. She needs to get over her fear of Serena to be the very best against Serena but I think that is close to impossible that is if Serena has no injuries or anything. I'm a big Williams sister fan, even when I watch Serena and she is psyching her self up when she is playing even im scared of her and im watching not playing her. With her tennis abilities and her mental strenght she is the best, but lets watch the game between her and henin only that will tell.