Definition of challenge round in English:

challenge round

noun

historical Tennis
  • The deciding round of a championship in which the reigning champion, who receives a bye to this round, plays a challenger who has qualified for this round by winning a knockout tournament.

    The Davis Cup, the last major tennis championship to be decided by a challenge round, adopted a standard knockout format in 1972.

Origin

Late 19th century; earliest use found in The New York Times.