The US star will start as the clear favourite for gold after equalling her own world record in the heats - which she wrested from Simmonds - in 1:14.74.
However, 17-year-old Simmonds has already claimed two golds and one bronze medal and tonight she has a good chance of making her fourth trip to the podium after clocking the second fastest time of the morning (1:16.68).
Both eased into the final and Arlen cannot wait to go up against her rival on what promises to be an electric final night of Swimming action at London 2012.
She said: 'This is my last race and I'm fired up and ready to go. I love the 100 and I'm just going to go all out in the final.
'I love racing Ellie. It's great and she's a sweetheart. It's sometimes hard when people are cheering against you, but it's really an energetic crowd and it pushes us all on.'
Meanwhile, in the women's 100m Breaststroke - SB9, Sophie Pascoe kept herself on course for a sixth London 2012 meal as she moved into the final.
The New Zealander clocked a new Oceania record of 1:19.28 to record the second fastest time of the heats, although in a close finish she was pushed all the way by Great Britain's Harriet Lee.
Both will face rivalry from fastest qualifier Khrystyna Yurchenko of the Ukraine, who clocked 1:18.61.
'I only race my own race. I just wanted to go out there and have a comfortable swim and that's exactly what I did,' said Pascoe.
'Over the past four years the field has grown and the standard is huge in general, but in this race in particular.'
Elsewhere, Brazilian Daniel Dias - already with five golds to his name - put himself in contention for yet another title by qualifying over two seconds faster than his closest rival in the men's 100m Freestyle - S5, while China's Yang Bozun was fully seven seconds faster than the field in the men's 200m Individual Medley - SM11.