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 2009 he reached the quarterfinal at the
German Open in Hamburg. He lost to Spaniard
David Ferrer. At the
2009 US Open he defeated
Giovanni Lapentti in the first round but lost to eventual finalist
Roger Federer in the second round. In his next tournament, the
BCR Romania Open in Bucharest he reached the semifinal where he lost to
Juan Mónaco. Greul finished the season ranked No.59. The year was highlighted by a win at an
ATP Challenger Tour Event in Todi, a semifinal appearance in Trani and Genoa. It was the best season in his career earning $305,269 and a match record of 15–15 in
ATP World Tour level. In March 2010 Greul was nominated for the
Davis Cup first round match against France. Greul defeated
Jo-Wilfried Tsonga in a
dead rubber. That was Germany's only win in this tie. 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. This was followed by a first round exit at the
2010 Sony Ericsson Open in Miami. 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. He reached the quarterfinal at the
2010 Ordina Open in s´Hertogenbosch and at the
Mercedes Cup in
Stuttgart. He could not defend the quarterfinal points in
Hamburg of the season before which meant a drop in the ranking. At the
BCR Romania Open he was knocked out in the second round by
Juan Ignacio Chela.
Doubles: 1 (1 runner-up) ==ATP Challenger and ITF Futures finals==