Groothuis, a specialist in the middle distances (1000 and 1500 metres), had his international breakthrough in 2005. In that year he reached a second spot during the Speed Skating World Cup meeting in Salt Lake City competing in the 1000 metres. This earned him a nomination for the 2006 Winter Olympics in Turin. During the Dutch Single Distance Championships in December 2005, he switched this nomination into a qualification by placing fourth behind Jan Bos, Erben Wennemars, and Beorn Nijenhuis, who all qualified as well.
In January 2006 Groothuis surprisingly became the Dutch national sprint champion, finishing first in two 500 metres and two 1000 metres races, outsprinting common names like Gerard van Velde, Nijenhuis, and Wennemars. A month later at the 2006 Winter Olympics, Groothuis would finish in eighth position in the 1000 metres. His time of 1:09.57 was 0.68 behind the winner Shani Davis.
In the 2010-2011 World Cup season, Groothuis would win the 1000 metre overall World Cup, dethroning perennial winner Shani Davis. He would also lead the 2011-2012 1000 metre standings going into the final race, but would be beaten out in the season finale by Davis and also just edged for the season 1000 title by Davis as a result.