The Zimbabwean hitman opened the scoring on the half-hour mark, nodding home a cross from Bandile Shandu.
Tashreeq Morris thought he had equalised in the 54th minute, but his effort was chalked off for an adjudged foul on goalkeeper Bongani Mpandle.
Rusike then got his second goal in the 68th minute with a composed finish, having also netted in the club's 1-0 win over Mamelodi Sundowns in the MTN8 quarter-finals.
United, who share the summit with Cape Town City with six points from two matches, have picked up from where they left off under coach Fadlu Davids last season.
Davids' men eliminated African champions Sundowns from the MTN8 and dispatched Platinum Stars with a 2-0 scoreline in their first league game of the season.
Ajax are second-from-bottom on the log, with Stanley Menzo's side drawing 1-1 with Golden Arrows at home in their 2017/18 opener.