@Zebrev
Even if Federer wins the Tour Finals, it is mathematically impossible for him to get the #1 ranking back after the Australian Open without winning the Australian Open. Here's why:
1. Best-possible year-end tallies from his point of view: 1. Nadal: 9,985 points, 2. Djokovic: 9,145 points, 3. Federer: 7,490 points.
2. Federer is not playing any Australian Open warmups, nor is he defending any points during those weeks, so he'd be on 7,490 points when the Australian Open began. Nadal is also not defending any points, so his minimum total is 9,985 points. I'm not sure whether he's playing any warmups to add to his tally but let's make the case as positive for Federer as possible and assume that he's not doing and so will be on 9,985 points. Djokovic is defending 90 points in Doha, so best-case scenario for Federer sees Djokovic lose them and be on 9,055 points heading into the Australian Open.
3. Best-case scenario for Federer at the Australian Open other than him winning it: he is runner-up; Djokovic and Nadal both lose in round 1.
4. In this case, points tallies after the Australian Open: 1. Nadal: 8,785 points, 2. Federer: 8,510 points, 3. Djokovic: 7,055 points.
Conclusion: Federer is not getting back the #1 ranking without winning the Australian Open.
His chances of getting it back in February or March aren't great, either, as he won Dubai and Miami and lost the final of Indian Wells this year, so he could gain at most 400 points unless he also plays and wins Rotterdam, but that's a rather unlikely schedule. And Nadal has a good chance to pick up points in those months.