Mariusz Fyrstenberg (born 8 July 1980) is a left-handed Polish professional tennis player whose speciality is in doubles.Marcin Matkowski and Fyrstenberg qualified as the eighth and final contenders in the 2006 Tennis Masters Cup. This was mainly because of their best Grand Slam result, in which they reached the semifinals at the Australian Open. In 2011, they reached the US Open men's doubles final and the final of the year-end championships.