Quad fatigue could indicate one of several things:
- Saddle is too far forward: frequently this strategy is used by triathletes to put the burden of the bike ride on the quads to avoid fatiguing the hamstrings before the running component. This is why Tri bikes have such steep seat tube angles and forward saddle positions
- Saddle is too low: This causes over-extension of the quads (overbending of the knee) so your initial muscular contraction is higher in tension but lower in value.
Try moving the saddle to a more aft position. This will place more work on your hamstrings and less on the quads, and may create a more balanced workload for the two groups.
Also, if you do not already have them, clipless pedals (I ride Speedplays) will reduce the amount of work done by the quads since you will be able to use more muscle groups during the entire pedalstroke instead of simply mashing the quads on the downstroke.
You might consider seeing a professional bike fitter if you have a new bike and it is giving you problems. You should be able to get a very good bike fitter for about $100. Anyone charging more than that is just trying to use a high price to make their fitting look "better."