Step 1: What is asked.
We need the first tennis player to win 400 Grand Slam singles matches.
Step 2: Why it matters.
Grand Slams are the four biggest tennis events. Reaching 400 match wins shows a very long and great career.
Step 3: The record holder.
Novak Djokovic became the first player ever to reach 400 Grand Slam singles match wins.
Step 4: Remove others.
Carlos Alcaraz, Jannik Sinner, and Lorenzo Musetti are younger players who have not reached this mark first.
Step 5: Confirm.
His long career across all four majors put him ahead of everyone for this milestone.
Step 6: Conclusion.
So the player is Novak Djokovic.
Answer: Novak Djokovic