You have come up with a great question and it's not really a matter of deserving it, it's more like earning it. It's funny how it came down to the last championships of the year to determine who is going to be the number one for this year, be it Roger, Serena or Bryan Brothers.
Serena Williams- She won the Australian Open, Wimbledon, and Year End Championships and would have won the USO except that she ran into a hot Kim Clijsters who trained like crazy to win the USO. Serena also did fairly good at smaller events like reaching the semifinals but what matters is how you do at the grandslams because they were 2000 points each. However, Serena will have to do much better at the smaller events next year if she wants to keep the number one ranking because Justin Henin is coming back, Kim Clijsters wants more slams, Wozniacki and Radwanska sisters are on the rise.
Roger Federer- He did good by winning the French Open and Wimbledon and he gained a lot of points at those two events which were enough to bolster him to the number one position. Also, his win at Cincinnati masters led him to the top because he had lost there in the first round last year and the masters events are 1000 points each this year so it was an advantage for him. However, next year is going to be different and he has to work hard to keep the number one position. Rafa would like to regain the lost ground, Djoko would want to leap ahead, Andy Murray would like to win a slam which means a lot of points and Del Potro looks like an emerging force, not to mention the dangerous Soderling and Cilic.
This year has been full of surprises but Roger and Serena have managed to hold their own and Congrats! to them for being the number player for the year 2009.