HONG KONG — The Rugby World Cup, which opens Friday in Tokyo, is likely to be about which of the 19 other teams has the right stuff to knock the two-time defending champion New Zealand All Blacks off their throne.

New Zealand slipped slightly in August. And after a loss to Australia and a combination of other results in Europe, the Kiwis were nudged from the top of the World Rugby rankings for the first time since November 2009. For three weeks, they sat a fraction of a point behind Wales, but regained the top ranking after Wales dropped a World Cup warm-up match at home to Ireland, 22-17, on Aug. 31 that dropped the Welsh to fourth.

A week later, on a day New Zealand thrashed Tonga’s World Cup team, 92-7, Ireland still slipped past the All Blacks by beating Wales again to go into the World Cup as the new No. 1.

Most experts, however, still make New Zealand a heavy favorite to secure a third straight World Cup title, and a fourth over all. But there are formidable challengers.