A try by Andrew Amonde midway through the second half earned Kenya a come-from-behind 29-22 victory over Namibia, who represented the continent at the past four editions of the quadrennial tournament.
Loose forward Jacques Leitao snatched four tries as Zimbabwe turned a seven-point half-time advantage into a 57-22 triumph over Madagascar before a 35,000 crowd at Stade Mahamasina.
Each country plays its three opponents once with the mini-league winners joining title-holders New Zealand, Argentina, Tonga and Georgia in Pool C.
The runners-up visit Siberia for a repechage semi-final against Russia and the victors tackle Uruguay or Hong Kong for a Pool A slot beside England, Australia, Wales and Fiji.
Pre-tournament favourites Namibia started impressively with two Heinrich Smit tries inside 12 minutes and Kenya did not score until the final minute of the half when Humphrey Kayange touched down.
But further tries from Michael Okombe and Edwin Achayo lifted the Kenyan Lions into a 19-12 lead six minutes into the second half.
Theuns Kotze converted a Chrysander Botha try and kicked a penalty to give Namibia the advantage again only for Kenya to score twice within five minutes.
Lavin Asego potted a penalty and converted the Amonde try to raise hopes of a first Kenyan appearance at the World Cup.
A much heavier Zimbabwe pack helped build an early 22-point lead before Madagascar scored two tries and narrowed the deficit to 22-15 at the break.
But the second half belonged to the visitors, who crossed the try-line five times and boosted their chances of returning to the rugby union showpiece after a 24-year absence.
Namibia play Zimbabwe and Madagascar meet Kenya on Wednesday in the second series of matches.