No. 10 France, runner-up last month in Vancouver, thumped invitational side Portugal 40-7 in the other Pool B match.
The Hong Kong Sevens continues this weekend, live on Sky Sports Action, with England seeking to continue their good start against Spain and USA.
Douglas now has 133 career tries for Canada.
With the victor of the 12-team qualifier being promoted to next season's HSBC World Rugby Sevens Series, the Cheetahs needed to win at least two matches in the pool stages to progress to today's knockout stages.
The Cheetahs were on the backfoot nearly immediately, as they struggled in defence, allowing Tonga to run in three tries in the first half, while managing one conversion to take a 17-0 lead at the break.More news: Alexandria Ocasio-Cortez: Campaign Finance Complaint Is 'Bogus'
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Australia are licking their wounds after losing 40-19 to New Zealand in their opening match of the "Pool of Death", with Fiji and Kenya their opponents on Saturday.
England's Dan Norton became the leading try scorer in the history of the tournament as he helped his team beat Wales 36-19, breaking the record set by Collins Injera at the 2017 event.
The loses came a day after they had lost 22-5 to defending champion Fiji, which was a stark contrast of last year's Hong Kong Sevens where Fiji and Kenya met in the Cup final with the Pacific Islanders winning 24-12.
Scotland's rugby-playing women, assisted by three from the Borders, have reached the quarter-finals of a sevens tournament in the Far East.