The 33-year-old South African, who has won his first event of the calendar year in four of the last five years, carded seven birdies at Doha Golf Club.
Spaniard Larrazabal, 32, also made seven birdies in a bogey-free round.
Andrew Johnston is a shot off the lead following a round of 66, with fellow Englishman Tommy Fleetwood and Scot Paul Lawrie both on five under.
Defending champion Branden Grace from South Africa and 2014 winner Sergio Garcia from Spain both shot 70s.
Louis Oosthuizen shot a seven-under-par 65 to share the first-round lead at the Qatar Masters with Pablo Larrazabal.