Merklein was especially successful in doubles. With various partners, he won four titles on the ATP Tour and reached rank 37 on 12 January 2004 his highest placing in the double world rankings. In addition, he was in three other finals. At Grand Slam tournaments in the second round at the Australian Open 2003 collection was his best result. In the Davis Cup, he won 15 of 27 matches for itself, eight in doubles and seven in singles.