infuse your top soil with some nice compost and nutrients and re-lay sod at this point - that is such a small area to fix and will cost you probably less than 100$ for the sod and soil treatment. If the issue has happened repeatedly (you've had to fix this area before) then I'd look at some environmental factors; watering patterns (time of day, how much you're watering, etc) and how much sun that area is getting compared to other areas of the lawn.