Lets look at what it will really take just to get to #1. The minimum assumption is Nadal defends all his points through Wimbledon in 2020. That is 6225 points.
Rest of 2019:
We'll assume WTF is a wash between the players and that Nadal skips Paris:
Moscow - wins it and 250 points
Vienna - no bye, skips (wins if he plays, but then loses final in Paris so points close, plays if he goes out early Moscow to Rublev in QF)
Paris - 1000 point win, which seems reasonable as Djokovic slightly beatable right now
The following super imposes results over 2020 schedule:
27 / 2019 Wimbledon SF 720
R32 Grass David Goffin Daniil Medvedev 4-6 6-2 3-6 6-3 7-5 H2H
25 / 2019 Queen's F 300
SF Grass Gilles Simon Daniil Medvedev 6-7(4) 6-4 6-3 H2H
24 / 2019 Stuttgart - F 150
R16 Grass Lucas Pouille Daniil Medvedev 7-6(6) 4-6 6-2 H2H
22 / 2019 FO - RG QF 360
R128 Clay Pierre-Hugues Herbert Daniil Medvedev 4-6 4-6 6-3 6-2 7-5 H2H
20 / 2019 Rome Masters - QF 180
R64 Clay Nick Kyrgios Daniil Medvedev 6-3 3-6 6-3 H2H
19 / 2019 Madrid Masters - QF 180
R64 Clay Guido Pella Daniil Medvedev 6-2 1-6 6-3 H2H
17 / 2019 Barcelona - SF 180
F Clay Dominic Thiem Daniil Medvedev 6-4 6-0 H2H
16 / 2019 Monte Carlo Masters - QF 180
SF Clay Dusan Lajovic Daniil Medvedev 7-5 6-1 H2H
12 / 2019 Miami Masters - SF 360
R16 Hard Roger Federer Daniil Medvedev 6-4 6-2 H2H
10 / 2019 Indian Wells Masters - SF 360
R32 Hard Filip Krajinovic Daniil Medvedev 6-3 6-2 H2H
9 / 2019 Dubai - 300 points final
R32 Hard Richard Berankis Daniil Medvedev 6-3 6-3 H2H
7 / 2019 Rotterdam - 500 point win
SF I Hard Gael Monfils Daniil Medvedev Jo-Wilfried Tsonga 46 63 64 H2H
6 / 2019 Sofia - skips
F I Hard Daniil Medvedev Marton Fucsovics 6-4 6-3 H2H
3 / 2019 Australian Open - SF 720
R16 Hard Novak Djokovic Daniil Medvedev 6-4 6-7(5) 6-2 6-3 H2H
1 / 2019 Brisbane - replaced by ATP Cup, 180 in group, 120 for quarterfinal win or plays Adelaide/Auckland - 300 points
F Hard Kei Nishikori Daniil Medvedev 6-4 3-6 6-2 H2H
The above alone is 6150 points, just 75 points off the goal post. It assumes he plays to his seed for the most part. It removes Sofia from schedule which probably will be on his schedule, but might be dropped if Medvedev makes SF at Australian Open. This would have him within 75 points. It also gets much easier once Medvedev passes Djokovic for #2 as that means Nadal guaranteed to seed out with Djokerer SFs at every major event. If Medvedev continues to cruise towards WTF he'll probably be within at least 1500 points of Djokovic and a good chance to pass him before IW/Miami swing and grab the #2 seed.
Nadal could easily have better results for the first half, but that assumes he stays healthy the whole period, etc., etc. and I'd bet against that. In short its very doable unless Medvedev completely derails due to injury or mental fatigue. Medvedev also does not have a ton of ATP 250/500 points which I failed to adjust for in the above. Med has Final in 500 at Washington and 500 for winning Moscow and St. Pete. Monte Carlo will count as one of those so he has 5 250/500 in the above tally so he loses 450 points right there.
This means he needs one big result like a final in a major or win Masters 1000 at IW/Miami to close the proposed gap.
