Juan Martin Del Potro: One of the biggest has beens of tennis.

Having started playing tennis at the age of seven, Del Potro won his first senior match in 2004 at the age of 15. In 2008, he became the first player in ATP history to win his first four career titles in as many tournaments.[5] He also completed the second-longest winning streak in 2008 and the second longest by a teenager in the Open Era, behind Rafael Nadal—with his winning sequence spanning 23 matches over five tournaments. He achieved a top-10 ranking by the Association of Tennis Professionals for the first time on 6 October 2008.
The highlight of his career came in 2009 when he captured his first Grand Slam title at the 2009 US Open, defeating Nadal in the semifinal and Roger Federer in the final—the first man to defeat them both in the same Grand Slam tournament. This helped him reach a then-career-high ranking of World No. 4. But soon on its heels came the low when he had to withdraw from most of the tournaments in 2010 due to a wrist injury.
Thereafter, started a series of highs and lows, from which his career could never recover. In the year 2011, Del Potro finished the year ranked world no. 11, despite being ranked no. 485 at one stage. He was named 2011 ATP Comeback Player of the Year. In the year 2012, he displayed some of his finest performances by winning the bronze medal at the London Olympics and ending the year ranked world no. 7, with a 65–17 win-loss record and four titles captured throughout the season. But just when we thought, he had left the ghost of his injuries behind, his wrist injury came back to haunt him in the 2014 season due to which he has now withdrawn from the 2014 US Open. 
Del Potro is one of only three players other than the Big Four to have won a men’s Grand Slam singles title since 2005, the others being Marat Safin and Stanislas Wawrinka. He became the second Argentine and the fifth-youngest man to win the US Open title in the Open Era. People were easily betting on the man who would go on to create history. But he never could reach his potential due to all the injuries that he has been struggling with over the years.
Like Nadal, who also has withdrawn from the US Open due to injury and whose constant battle with injuries might be preventing him from becoming the Greatest Player of all time, Del Potro too may have his injuries to blame for him not reaching his true potential. He has constantly proven himself by bouncing from every injury, but at a time when there are so many talented players around and competition is extremely fierce, even a single injury could potentially destroy one’s career. That he has managed to still stay among the top players even after so many injuries is a salute to him, but at the end of the day, Del Potro will not be the one remembered for his spectacular wins.