Ferstl took part in FIS races and national championships for the first time in the 2003/2004 season . At the European Youth Olympic Festival in January 2005 in Monthey , he was fifth in slalom and eighth in Super-G. In the next month Ferstl took part in two European Cup races for the first time , after which he did not start again in this racing series until February 2007. However, it was not until March 2010 that he was placed in the top 30 for the first time and thus won European Cup points for the first time. In 2007 Ferstl took part in a junior world championship for the only time . He only finished 48th in the giant slalom and was eliminated in all other disciplines. Two weeks before this Junior World Championship, he made his debut in the World Cup in the two downhill runs from Garmisch-Partenkirchen . He was only able to place himself in the last or penultimate place and then did not appear in the World Cup for almost four years. Since 2011 he has been back in the World Cup sporadically, but initially remained without World Cup points.
In March 2010, Ferstl took second place behind Stephan Keppler at the German championships in the super combined, which were held in St. Moritz , Switzerland . In February 2011 he achieved his first top 10 result in the European Cup with a fifth place in the super combination of Sarntal / Reinswald . Eleven months later, on January 26, 2012, he celebrated his first European Cup victory on the downhill from Altenmarkt-Zauchensee . Ferstl won World Cup points for the first time on January 18, 2013, finishing 30th in the super combination of Wengen . On February 23, 2013, he finished in the top ten for the first time in the Kandahar downhill run from Garmisch-Partenkirchen. In the following 2013/14 season he was only able to classify himself twice in the points. While he was in the top 10 once in the 2014/15 and 2015/16 seasons, he did so twice in the winter of 2016/17.
After a cautious start to the 2017/18 season , Ferstl surprised on December 15, 2017 by winning the Super-G in Val Gardena (his best result so far was fifth). This was the first World Cup victory of a German in a speed discipline after 13 years ( Max Rauffer 2004 also in Val Gardena), in a Super-G it was even the first German victory after almost 27 years ( Markus Wasmeier 1991 in Lake Louise ). On January 27, 2019, he became the first German to win the Super-G in Kitzbühel (his father won the downhill in 1978 and 1979).