The question asks about the oldest man to reach the semi-finals of an ATP Masters 1000 event in 2025. The given options are Novak Djokovic, Andy Murray, Rafael Nadal, and Roger Federer. Let's analyze each option to determine the correct answer.

1. Novak Djokovic: As of 2025, Novak Djokovic is a prominent tennis player who has continued to compete at a high level even as his age advances. He is known for maintaining excellent physical condition and has been breaking records in tennis history for several years.
2. Andy Murray: Andy Murray, a successful tennis player from Britain, has faced significant challenges with injuries in recent years. These have somewhat limited his ability to consistently reach the late stages of major tournaments post his peak years.
3. Rafael Nadal: Rafael Nadal, a legend of the game, particularly on clay courts, has experienced several injuries but is known for remarkable comebacks. While he can still perform exceptionally, Djokovic's form has generally been stronger in the later years.
4. Roger Federer: Roger Federer had been semi-retired and eventually retired from professional tennis before 2025, which rules him out from achieving this feat in that year.

Considering the steady high performance and continued success of Novak Djokovic from recent records, he is the most likely to have achieved this milestone of being the oldest to reach the semi-finals of an ATP Masters 1000 event in 2025.

Conclusion: The correct answer is Novak Djokovic. He maintained his competitive edge and physical fitness to reach significant stages in major tennis events, compared to his peers listed in the options.