I recently bought the above mentioned lens locally for my Nikon D5100 and it's much sharper and has more contrast than the standard kit lens.

I seem to have focus issues with the aperture wide open especially in low light situations. When using single point focus on a subject, the background is focus and the subject is blurry. I trying to work out if it is my technique or a lens issue. I'm more inclined to think it is a lens issue.
I only bought the lens in January so it is still under warranty.

Any thoughts?
Thanks in advance