KwaZulu-Natal’s Justus finished quickest in the 100 metres freestyle after posting a time of 50.92 seconds, narrowly outside the 48.93 Commonwealth Games qualifying time.
Border’s Erasmus flew through her 100m butterfly heat by clocking a time of 59.35, a fraction of a second slower than the required 58.89.
All swimmers at the six-day event being held at the Kings Park Aquatics Centre must record the stipulated times if they want to have a chance of making the Team South Africa squad that will compete in Glasgow 2014 later this year.
Justus, 18, upstaged team-mate Chad le Clos in the heats, although the Olympic gold medallist did reach the semis with the fourth fastest time of 51.00.
Brad Tandy (50.97) and Bradley Vincint (50.99) was sandwiched in between them.
Le Clos, who is bidding to win eight titles this week, also reached the 200m backstroke final where he was second quickest following a time of two minutes, 03.55 seconds (2:03.55).
Darren Murray from North Gauteng topped the qualifiers with a time of 2:02.46, while Christopher Reid (2:03.59) was third.
Erasmus, 18, was followed by Vanessa Mohr (1:00.85) and Tayla Lovemore (1:03.07) on the women’s front, with all three aiming to go quicker in the semis and finals of the meet, which is doubling up as the nationals and official qualifier for Glasgow, if they want to participate in the July 23 to August 3 spectacle.
Elsewhere, Franko Jonker was the top qualifier in the 200m breaststroke after posting a time of 2:30.02, ahead of Tatjana Schoenmaker (2:31.28) and Daniela Lindemeir (2:33.83).
Finally in the 800m freestyle, Caitlan Kat booked her place in the final as the fastest qualifier after a time of 8:58.39. Kyna Pereira (9:01.99) was second, followed by Carmen le Roux (9:02.23).