After overseeding some bald spaces with fescue in my centipede lawn, I first saw some fescue germinate under the straw. The fescue died for lack of water in July, then clumps of this grass began to grow and take over the spaces.
It looks like grass. It grows in clumps of a lighter green than centipede. It is hard to mow and seems to bend rather than be cut. Each stem ends in a small ball with three blades growing horizontally out of it.
your lawn weed is actually a sedge rather than a grass, and it is usually called spikesedge, but has had other names such as flatsedge and thimble sedge, or more specifically green kyllinga or false green kyllinga. It has been placed in either the big genus Cyperus or its segregate genus Kyllinga. I am not completely sure which species you have, but it is probably one of these: Kyllinga brevifolia, or Kyllinga gracillima. It is a turf grass weed that increases with moisture, and decreases with drought. There is a lot on the internet concerning control, such as: http://www.turffiles.ncsu.edu/weeds/Kyllinga_Green.aspx
Dr. Steven R. Hill, Botanist, SCNPS