His major breakthrough was at the 2006 Nasdaq-100 Open, where after beating Adrián García and Ricardo Mello in the qualifying rounds, he reached the round of 16 at the main draw, defeating Paradorn Srichaphan, Dominik Hrbatý, and Tim Henman before succumbing to fourth seed Andy Roddick 6–3, 3–6, 6–2.

In his next tournament, the BCR Romania Open in Bucharest he reached the semifinal where he lost to Juan Mónaco.

The year was highlighted by a win at an ATP Challenger Tour Event in Todi, a semifinal appearance in Trani and Genoa.

Greul reached his career high ranking of #55 after advancing to the third round of the 2010 BNP Paribas Open in Indian Wells after beating Richard Gasquet and Gaël Monfils.

At the 2010 BMW Open in Munich he was defeated by Marin Čilić in the second round after leading 4:1 in the deciding set.