The group took on When Love Takes Over by former X Factor judge Kelly Rowland on tonight's latest live show, which was a club classics special.
Louis Walsh said: "Guy it's working, the image is coming together. The public love his band and this band has a big future!"
Gary Barlow said: "I think we're witnessing the birth of a new boy band here, but maybe a bit more harmony next week would be good."
Nicole Scherzinger said: "You're very cool and effortless when you perform, but I need more energy from you. We need a bit more harmonies and energy."
Tulisa said: "I knew I saw that potential in you, each week you are getting better and better and better and your female fanbase is getting stronger and stronger. That was a wicked performance tonight."
Watch Union J on The X Factor below...
[cat c="the x factor"]XFactor UK[/cat] live shows continue at 7PM tomorrow with the latest results show.
As well as featuring the latest sing off and subsequent elimination, Sunday's evening's episode will also feature performances from JLS and Labrinth.