Knewstubb was tackled in the air taking the opening kickoff, causing America's Perry Baker to be sin-binned.
That result sent out some warning signs, and the Springbok Sevens were then well and truly outplayed by England in their Cup quarter-final as they plunged to a humbling 22-5 defeat.
The Fiji Women's Sevens team have managed to avoid the wooden spoon for finishing last at the World Series tournament in Dubai, thanks to a 17-5 win over Kenya.
There was to be no first Dubai title for U.S., though, as tries by Dylan Collier and Ngarohi McGarvey-Black ensured it would be New Zealand celebrating a long-awaited sixth Dubai title.
"Canada, they're just getting better and better", said New Zealand's Tyla Nathan-Wong, named player of the final.
Putin sends tanks to border as Ukraine bans Russian men from crossing
Putin noted that two Ukrainian Security Service officers were among the crew and were effectively in charge of the operation. But Moscow accuses Kiev of illegally entering its waters off Crimea, which it claims since annexing the peninsula in 2014.
The Australians were undefeated coming into day two and started with a comprehensive 38-0 victory over Argentina to breeze through the quarter finals on Saturday. When I then got the phone call telling me I had won it again I was truly shocked but when I thought about it I realised it wasn't about me, it was actually about my team mates and what they do to make the game so much fun and helping create opportunities to score all those tries.
New Zealand and England went unbeaten in winning the other two groups. However, Waisea Nacuqu had other ideas and his breakaway try levelled the scores, only for Vilimoni Botitu to step up and miss the straightforward conversion to mean extra-time would decide the victor.
The 21-7 victory over Scotland saw the team progress to the semi finals where they produced an inspired performance to overcome England. A crucial turn over in the last play of the game by debutant Scott Gregory secured the 7-5 victory, and was also awarded the UL Mark of Excellence as "Moment of the Tournament". The Canadian women have won 10 of 12 matches this season, losing only to the Black Ferns.
The men, whose pre-season preparations were interrupted by a labour dispute with Rugby Canada, now head to Cape Town for next weekend's second stop of the HSBC World Rugby Series.
A top-four finish this season means automatic qualification for the 2020 Olympics.
Earlier, Samoa set up a Challenge Trophy final against France.