During the US open final between Medvedev and Djokovic, at some point in the second set, on the 2-1 Djokovic and Medvedev serving, Medvedev missed his first serve giving Djokovic a chance to break him on his second serve. After the second serve, in mid play, apparently some music was played and the umpire ruled to start again, giving Medvedev a second first serve. Why didn’t the umpire rule to replay on Medvedev second serve, since the first serve had already been missed even before the music was played? See here for reference
In ITF rules of tennis is written: "If a player is hindered in playing the point by a deliberate act of the opponent(s), the player shall win the point. However, the point shall be replayed if a player is hindered in playing the point by either an unintentional act of the opponent(s), or something outside the player’s own control (not including a permanent fixture)."
In US Open final, Medvedev was hindered in playing the point by music, so the point was replayed. Each point starts with the first serve, that's why he was serving his first serve again. It is fair as players are more nervous at second serve and in this situation player on the serve would be clearly out of rhythm.