Ellis led the United States to World Cup glory in 2015 and 2019, but the Americans were made to look anything but champions as they squeezed into the knockout rounds.

Last year's European Championship runners-up Germany, Brazil, Italy and Olympic champions Canada were all turfed out in the group phase. Morocco, Nigeria, Jamaica and South Africa instead all progressed.

"Whoa, it's been a doozy so far," the 56-year-old Ellis told reporters.

"I think we all feel this -- that gone are the days of total predictability.

"We've seen giants of the game knocked out, we've seen debutants advance to the next round, which lends itself to this being one of the most interesting, unpredictable and arguably exciting World Cups we've seen to date."

Looking into the reasons for the success of the lower-ranked teams, Ellis -- now head of FIFA's technical study group -- said: "Overall the level of play is advancing.

"The level of coaching has advanced clearly and we're seeing stars that we've never seen before on this kind of stage."

Better defending and goalkeeping were chief among the reasons for the gap closing on the teams traditionally regarded as the best, Ellis and her FIFA colleagues said, digging into the data from the group stage.

The last 16 begins on Saturday when Switzerland face Spain and Japan play Norway.

Holders the United States meet Sweden on Sunday.