He made his debut in the Great Britain Davis Cup team in 2009, losing against Ukraine, but then won eight successive matches to help Great Britain into the World Group. He also won his doubles match in the World Group quarter final against Italy. At the 2010 Commonwealth Games, he won the mixed doubles gold medal with Jocelyn Rae for Scotland.

He has reached nineteen ATP Tour doubles finals in his career, winning eight of them: two in 2009, 2012 and 2013 and one in 2011 and 2015. In 2011, he had his best doubles Grand Slam results, reaching the quarterfinals of Wimbledon, and then two months later equalling it at the US Open.

He has had a number of different partners, but primarily plays alongside his British compatriots, most notably Ross Hutchins, Jamie and Andy Murray, Ken Skupski and Jonathan Marray. Fleming's most successful partnership has been with Ross Hutchins, however whilst Hutchins was off the tour with illness, Fleming spent most of 2013 partnering with Marray.

He retired from professional tennis in January 2017, to take up the new position of national coach for Tennis Scotland.

Fleming first picked up a tennis racquet as a toddler and copied his older siblings by playing for a local club in Linlithgow. When he was eight, he began playing in mini tennis tournaments. Working his way up through the ranks Colin loved to compete, and regularly travelled long distances to play in regional challenge events and regional and GB junior tournaments. He was never at the top rank in the juniors, being small and slightly built until late teens, but held his own through court craft and determination. He also began his county career as a junior, representing North of Scotland, culminating in the great enjoyment he has in playing Summer County week and the camaraderie that he has with the North of Scotland players. As a teenager, he played in and around Edinburgh for the Blackhall club men's team. Brian Barnet coached Fleming up until he was about 14 when Fleming moved to the newly opened Next Generation club in Newhaven where Judy Murray began coaching.

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Wikipedia