Georges St-Pierre fought his fight. When Nick Diaz wanted to stand, exchange punches and force a little offense on his end, St-Pierre would simply take Diaz down, establishing his ground game and controlling the pace of the match. Over time, Diaz became so concerned with defending the take-down and played right into GSP's hands throughout the entire fight.
In the end, GSP easily defeated Diaz by unanimous decision (50-45) and retained the UFC Welterweight title in the main event of UFC 158.