Entered USA on VWP; want to visit Canada, then return

There is no official rule preventing you from doing this, but for non-residents of Canada, the VWP clock continues while you're there in order to prevent visa runs.

However, given how long you'll be in Canada for, it should not be a problem to re-enter if you can present a confirmation of the return flight as well as paperwork proving you'll attend the Mayweather and McGregor fights. You may not be given a full 90-day period of stay though.

Maybe? I got a new 90 days having only been in Canada for four. But I didn't need it (my journey was New York - Niagara Falls NY - Toronto - Chicago - Long Beach - San Francisco, over a period a bit over two weeks). I guess if you need it they might be more sceptical.

I did actually have drama re-entering the US at Toronto Airport. The computer was fine, and (I think) was the one to give me the new 90 days. My original entry stamp at JFK was wrong though (the wrong year). I even asked the CBP people at JFK baggage claim and they told me not to worry. Maybe I should have insisted.

By the book, re-entry from Canada should not give a new 90 days. It MIGHT (and did in my case). Do NOT rely on that.

If https://i94.cbp.dhs.gov/I94/ shows your exit from Canada, and especially if you re-enter the US by air and use an Automated Passport Control machine, you MIGHT get a new 90 days (as I did). No promises.