He reached the semifinals of the 2006 Australian Open and won a silver medal in men's doubles with partner Rainer Schüttler at the 2004 Athens Olympics.

While facing Sébastien Grosjean late in the fifth set of a marathon match, Kiefer threw his racquet midpoint.

The umpire Carlos Bernardes said he did not believe the act was intentional and noted Grosjean had already hit the ball before the flying racquet could have had any effect on his shot.

Kiefer injured his wrist while playing at the 2006 French Open, and announced his return on 5 July 2007, having fallen to the 404th position on ATP.

Kiefer returned at the 2007 Gerry Weber Open, losing in the first round to eventual champion Tomáš Berdych.

In the first match, he lost against Australia's Lleyton Hewitt, who had been six months inactive due to an injury.

In the second singles match, Kiefer lost again, this time to USA's James Blake.

Nevertheless, Kiefer won both of the doubles matches with Sabine Lisicki against both Australia and the United States.

In the 2009 Sony Ericsson Open in Miami, Kiefer beat "the magician" Fabrice Santoro in the second round.

At the 2009 Monte-Carlo Rolex Masters, Kiefer lost in his first match against qualifier Andreas Beck.

At the 2009 Internazionali BNL d'Italia in Rome, Kiefer lost again in his first match against Juan Mónaco in straight sets.

Kiefer suffered from back problems which eventually made him lose against Jérémy Chardy in the next round.

Kiefer then participated at the 2009 French Open in which he beat qualifier Ilija Bozoljac in four sets.

He did so in the doubles match with Mischa Zverev against Spain's Fernando Verdasco and Feliciano López.