Ms. Hou earned the right to challenge Ms. Ushenina this year by winning the 2011-12 Women’s Grand Prix, a series of six tournaments featuring most of the world’s best female players.

Though Ms. Ushenina is the reigning champion, she was the underdog at the start of the match, as she is ranked No. 17 in the world among women, and Ms. Hou is No. 2. The way that the match has unfolded has underscored a recurrent criticism of elimination tournaments as a method for selecting a champion: the best player does not always win.

An exceptional feature of the current match is that Ms. Hou has won all of her games when she has played Black, which is usually disadvantageous, because White moves first.

Ms. Ushenina’s problems have not been in the opening phases of the games, where she has generally achieved small advantages. Rather, she has consistently made miscalculations later in the games, or missed possible moves in complicated positions.

That is what happened on Wednesday. She blundered on Move 33, which allowed Ms. Hou to push a pawn, attacking Ms. Ushenina’s queen. That pawn kept moving forward, and when Ms. Ushenina resigned on Move 40, it was because the pawn was about to promote to a queen, after which Ms. Hou would have had an overwhelming material superiority.