The capital city club - no longer with a for sale sign outside - followed up their win over the Scarlets by edging out the Ospreys 24-21 at the Arms Park.
It was only achieved by holding on after a dramatic fightback, to strengthen their position in the race for a top-eight finish in the United Rugby Championship.
Having seen his side come close to letting another commanding lead slip, Van Zyl admitted the manner of the finish tested nerves — but the overall outcome across the two fixtures is what matters most.
“It’s definitely better to win from a comeback than to lose from being in front,” said van Zyl.
“Honestly, on 24-7, I looked at the time and it was 66 minutes. I said, is this possible? So I shouldn’t have said it!
“We were actually OK. I think we had decent pressure on them but we lost the ball a few times and then gave them the chances at the end.
“But if you look at the two derbies, we played two games and we got nine points from it. If you asked me before, I would have taken it definitely.”
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Cardiff appeared to be cruising when they surged into a 24-7 lead midway through the second half.
Callum Sheedy’s early penalty had set the tone before Rory Thornton crossed in the first period, though the visitors stayed in touch thanks to Kieran Williams finishing off a sharp break from Dan Edwards.
Sheedy’s boot kept Cardiff ahead at the break, and they pulled clear after the restart.
Liam Belcher powered over from close range before Dan Thomas added a third try, both converted by Sheedy, to establish what looked like a decisive advantage.
But, as had happened the previous week, the contest swung late on.
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The Ospreys refused to fold, with Cam Jones breaking through to tee up Jack Walsh, and Edwards converting to narrow the gap.
When Edwards then crossed himself and added the extras, Cardiff’s lead was cut to just three points heading into the closing moments.
This time, however, they held firm.
Van Zyl was not surprised by how tight the encounter proved, pointing to the nature of derby fixtures and the occasion itself, played in front of more than 12,000 supporters in bright conditions at the Arms Park.
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“It’s always close isn’t it. The derbies stay close and it was good for the crowd all the way to the end.
“The game started with some fireworks for the run out and when the sun is out , it helps the whole atmosphere. It was brilliant today.”
While the victory represents a major boost to Cardiff’s ambitions, Van Zyl remains focused on maintaining momentum rather than looking too far ahead.
“Hopefully we will get a few players back now and it’s the same thing. The next game is the most important one.
“Glasgow are a quality team but we will go there and give it a good shot. Reflecting on last season, it was almost like we were chasing from behind and trying to get in there, whereas we made it a point to just focus more on week to week, trying to get points where we can.”
That approach has delivered tangible results over the past fortnight.
Two intense derby tests have yielded nine valuable points — and, for Van Zyl, that return could prove decisive as Cardiff continue their push towards the play-offs.
The Ospreys' own faint play-off hopes are now over and head coach Mark Jones had no complaints.
The former Wales wing felt his team had not handled their periods down to 14 men with the same calm authority as in previous matches.
"I don't think we managed those periods anywhere near as well as we have done in the past," said Jones.
"Tthat led to us probably burning a lot of energy in our own half, feeding Cardiff, really, when we didn't need to. So a good bit of learning for us there.
"But I have to say, I thought Cardiff were just a little bit quicker to the punch everywhere, if I'm honest with you.
"They just looked a little bit sharper, a little bit cleaner on things. They executed a bit better. I think that's probably why they've won so many tight games this year, because of their accuracy.
"We would have loved to have delivered more of what we have done over the last month, but we didn't reach that level tonight."
