But here are more data (not mine):
Federer's career is 5 years ahead of Novak's. Fed has 252 wins in slams by now, Novak 141. That is 111 more, divided by 5, it's ~22. 4 slams per year, maximum 28 wins. So just by reaching semi in every GS, in 5 years he will surpass him. He does not have to win a single GS to achieve this!
Similarly, Rafa's career is 1year ahead of Novak. The difference is 16 wins. Reaching semi in just 3 slams, and Novak will surpass this.
But, of the top 20 in that list, only Hewitt is from Fed's generation(!!!)
And from the current generation, there are 3 already, 2 of which already surpassed Hewitt!