Explanation:

- Option 1: Rafael Nadal

- Correct Answer: Rafael Nadal won the 2019 US Open Men's Singles title. He defeated Daniil Medvedev in a thrilling five-set match in the final.

- Option 2: Roger Federer

- Roger Federer, the Swiss tennis legend, did not reach the finals of the 2019 US Open. He was eliminated earlier in the tournament.

- Option 3: Daniil Medvedev

- Daniil Medvedev reached the finals of the 2019 US Open and put up a strong fight, but he lost to Rafael Nadal in five sets.

- Option 4: Novak Djokovic

- Novak Djokovic, another top contender, was unable to defend his title from the previous year, as he retired due to injury during his Round of 16 match.