Last week, the Principality Stadium told fans they will not be allowed to camp on its grounds in the run up to his two performances in the capital.
However, there was a row of tents in position along Westgate Street on Monday morning, with around 45 people waiting for the first show on Tuesday night.
Friends Tatianna PuertoReal, 24, from New York and Julia Parrey, 21, from Pennsylvania, have been queuing since lunchtime on Sunday.
The super-fans met at a Harry Styles concert and went to all the Wembley shows last week.
Julia, who is studying in the Netherlands, has seen him 15 times in total and Tatianna 17 times, but coming to the UK is furthest they have travelled for him.
It will be the first time that the One Direction turned solo superstar has performed in the Welsh capital without his former bandmates.
Lucie Bailey, 18, from Devon has seen all the UK dates of the tour and has spent around £7,000 on tickets and travel.
She joined the queue at 11am on Monday and is 45th in line but regret now getting there earlier.
Lucie said: "His concerts make you feel so safe and it's a place you can actually be yourself…there is no judgement, everyone is always really lovely.”
In a statement on Monday afternoon, the stadium said: "A polite reminder that for safe guarding reasons overnight camping is not permitted anywhere on the stadiums footprint. Temporary toilets and first aid provision for fans will be available at Gate 1, 3 and 6. Stay safe and hydrated in the warm weather folks! Diolch."
There will be major road closures across the city ahead of the concert which is over two nights starting tomorrow.
For everything you need to know about the set list, support acts and timings, click here.
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