Rafa is known for being injured and regular knee tendinitis preventing him from playing full calenders, so it'd be impossible for him to reach 300 weeks.
Its a miracle that he's reached 200 weeks, and a miracle that he's won 4 US Opens despite not playing in 2012 and 2014, and retiring in 2018 USO semis, and playing with a torn stomach muscle in 2009 USO semis.
He's on the brink of tying the US Open record and tying the overall slam record despite being consistently injured during his prime, and that has most likely cost him the 300 week milestone.
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Before Tour Finals Nadal can only add 640 points at the Paris Masters, or by playing 250/500 level events. And he's already 1010 points behind Djokovic (now Djokovic is in 4R of US Open). In addition, at Tour Finals Nadal will drop 200 points more than Djokovic, so he's 1210 behind in the race.
To get to YE #1 Nadal has to do very well at Paris/ATP Finals and hope that Djokovic won't add many points during the rest of 2020.
But Djokovic can add 1820 points at US Open, 400 in Rome (if he plays), 1280 at French Open, or by playing 250/500 level events. Plus ATP Finals.
Thiem is 3645 points behind Djokovic now, and could add 1910 at US Open, 955 in Rome, and 800 at FO, and 910 in Paris. But he'll drop 800 at Tour Finals.
It seems neither Nadal nor Thiem has a good chance at surpassing Djokovic in 2020.
Djokovic will reach 285 weeks on Monday 7 September. On 8 March 2021 he could reach 311 weeks. He might be at risk of losing the #1 ranking at the 2021 AO to Nadal, who only made a QF at 2020 AO, but if Djokovic does well in 2020 his lead will be sufficient to keep #1 until March, where he can add points at the sunshine double.
I think Nole will surpass the total of weeks of Roger, but I'm not sure about him being No. 1 until March.
In the rankings after Australian Open 2021, the things are like this (Points without WTF 2019, ATP Cup and Australian Open).
Nadal won't add points in this clay season, he could add points in Paris (360) and WTF.
Thiem still can improve his results in US Open, Rome (10), RG (1200), Paris (90), WTF.
Nole can improve his results in RG (720), Rome (600), WTF.
Medvedev can improve in US Open (1200), Rome (10), RG (10), Paris (10), WTF.
Points in parenthesis are their results of last year.
If Thiem wins this US Open he is going to have 7070 in the race for the rankings after AO. And still could add in another tournaments.
If Medvedev wins the US Open and have a decent clay season and great end of season, he could be around 8,000 in this race.
Djokovic his main chance is WTF, he have good results in RG and Rome the last year so he won't be able to add many points, could be crucial if he finds a way to do it.