In 1999, Greenhalgh, with partner Grant Silcock, won the Hong Kong Open. They defeated the experienced pairing of Mark Knowles and Daniel Nestor in the semi-final and won the final in a walkover, after one of their opponents, Andre Agassi, withdrew with a shoulder injury.[2] It would be his only title win on the ATP Tour and meant that he broke into the double's top 100 rankings for the first time. As a singles player, his highest ever ranking was 327, attained in 1995.
He regularly represented the New Zealand Davis Cup team during his career, participating in a total of 15 ties. In singles he had only a 2–9 record, but won 11 of his 13 doubles rubbers, which is a national record. His six doubles wins with Brett Steven makes them the most successful ever pairing for New Zealand in the Davis Cup.[4]