16 is for winning 50 or more matches in 16 different seasons, the most for a man in the Open Era. He achieved the feat every year from 2002 to 2012, then in 2014, 2015, 2017, 2018 and 2019.
17 is for reaching the final of 17 straight tournaments between 2005 and 2006. After falling to Nadal in the semis of Roland Garros in 2005, he reached the final at all six events he played the rest of the year (winning five) and then at the first 11 events he played the next year (winning seven). Andy Murray snapped the streak in the second round of Cincinnati in 2006.
18 is for his 18 year-end Top 10 finishes, which puts him in a three-way tie with Nadal and Djokovic for the most in ATP rankings history. He finished every year from 2002 to 2020 in the elite except for the aforementioned injury-marred 2016 season.
19 is for winning ATP titles in 19 different countries in his career. He won 103 ATP titles in all. The country breakdown: United States (23), Germany (16), Switzerland (11), the United Kingdom (10), Australia (8), UAE (8), China (4), France (3), the Netherlands (3), Qatar (3), Spain (3), Austria (2), Canada (2), Thailand (2), Italy (1), Japan (1), Portugal (1), Sweden (1) and Turkey (1).
20 is for his 20 career Grand Slam titles. He was the first man in tennis history to hit that number, and has since been surpassed by Nadal and Djokovic, who now have 22 and 24, respectively.