Buffalo Bills running back CJ Spiller has admitted that he was frustrated throughout the 2013 campaign.

The 27-year-old sustained an ankle injury early in the season and was not able to get back full fitness during the year, which resulted in his statistics regressing from the 2012 campaign.

Spiller told reporters: "It was very frustrating. I'm a very competitive person and I want to be able to be out there to help my team win.

"After I got hurt, I was anywhere from 50 to 60 percent. I was never myself."

Buffalo face the Chicago Bears in their 2014 season opener on Sunday.