This document presents a new approach to improve document clustering by exploiting the semantic relationships between terms contained in Wikipedia. The approach first maps terms within documents to corresponding Wikipedia concepts. It then calculates the semantic similarity between terms using Wikipedia's link structure. The document vectors are adjusted so that semantically related terms gain more weight. The approach differs from previous work by using a well-known measure of semantic similarity based on Normalized Google Distance, and by applying phrase extraction to more efficiently map terms to Wikipedia concepts. An evaluation on two datasets found the approach improved clustering results over other state-of-the-art methods.