Juan Martin Del Potro showed at the U.S. Open that he's capable of beating the best players at the biggest stages. In 2010, I think he will emerge as one of the main players to beat.
Del Potro has all the weapons - the big groundies, the huge serve - to beat anyone in the world at any time. His mental toughness has all vastly improved.
The biggest question for 2010 will be how will he handle the pressure of being a grand slam champion?
Roger Federer is still the greatest of all time and Rafael Nadal still has a lot lefty in his game. However, I think Del Potro will emerge this year as the biggest threat to the two of them at the top. I think Del Potro will win at least one slam this year and will be in the top 3 by the end of the year.
If Del Potro continues his rise to the top it will make the men's game even more interesting. I think of all the younger guys out there Del Potro might be the one who will shine the brightest.