Career
Herdling began his career with SG Heidelberg-Kirchheim and moved later to SpVgg Neckarsteinach in the youth ranks. At the beginning of the 2002-2003 season, he joined TSG 1899 Hoffenheim in the Regionalliga Südaughter He left Hoffenheim in the 2008-2009 season to join SV Waldhof Mannheim.
His first second division match was on 28 March 2008 in the match against Football Club Augsburg.
His first goal in the second division was on 13 April 2008 when he scored the only goal in the loss against Alemannia Aachen. In the third round of the DFB-Pokal 2003-2004 Herdling scored the winning goal for TSG Hoffenheim in the quarter final against Bundesliga side Bayer 04 Leverkusen.
Kai Herdling played in the first half of the season 2008-2009 for the Regionalliga team SV Waldhof Mannheim, but returned to Hoffenheim on 26 January 2009. On 7 March 2010, he made his Bundesliga debut against FSV Mainz 05 but otherwise stayed with Hoffenheim"s reserve team
On 16 April 2012, Herdling was loaned out to the Philadelphia Union of Major League Soccer, and played for two months.
In his time in America, he played in four league matches (starting three) and two Lamar Hunt United States. Open Cup ties. After he had scored twice in Hoffenheim"s first round cup victory against SG Aumund-Vegesack, coach Markus Gisdol took him to the first-team squad for the 2013-2014 season.