Federer is the second best player at the moment for a good reason. He has outperformed Murray in last 52 weeks, so he is ranked #2. He is almost 2000 points ahead of Murray.
well, the problem is that many of those points come from tournaments played nearly one year ago (Indian Wells-Madrid especially),
then what should reasonably be said is that Federer and Murray are very close at the moment, and Murray having a good chance to get ahead of the older guy but still having to do better in Masters 1000 tournaments and on clay for that, and Fed can still have his own word in that.
Federer is guaranteed the number 2 until the end of Indian Wells, but already after Indian Wells the situation is very tight : 8280 points for Fed, 8170 for Murray, although Fed will play in february and Murray will not.
Then Murray will play Miami but has to "defend" his last year's final.
And then Federer will lose his points from Madrid, and that's the best opportunity for Murray to get ahead of him, esp. as he will play Monte-Carlo and maybe Barcelona whereas Fed will not, but still they are very close after Madrid at the moment (7300 for Murray, 7235 for Fed ; after Roma : 7210 for Murray, 6875 for Fed)