Martin Maritz of South Africa carded a 5-under 66 on Saturday and is alone in second place at minus-9. Shaun P. Webster, who won this title in 1998 when it was part of the Third Level Hippo Tour, also shot a 5-under 66 in round three and finished 54 holes in third place at 8-under-par 205.
Klas Eriksson (68), Mikko Ilonen (66) and Raymond Russell (66) are knotted in fourth place at 7-under-par 206.
This is a dual European Tour/Challenge Tour event with most of the top stars of the European Tour over in New York for the U.S. Open at Winged Foot.
Nystrom parred his first eight holes and Ilonen matched him in the lead at minus-8. Nystrom birdied the par-5 ninth, a hole he eagled on Friday, to move one ahead at 9 under par.
Ilonen fell back with a bogey at the 13th, but got the stroke back with a birdie at 14. Nystrom, playing in the final threesome, birdied the same hole to get to 10 under, but Maritz, who tallied four birdies in a five-hole stretch around the turn, clawed within one thanks to a birdie at 15.
Maritz parred out, but Nystrom birdied the par-3 17th to re-establish his two-shot lead.
Nystrom has played well since the beginning of the month with a tie for seventh at the Wales Open. He has made three cuts in nine starts this season on the European Tour, but is still in search of win No. 1.
'There is a lot at stake,'acknowledged Nystrom, who can wrap up his playing card for next year. 'I will play well and have fun tomorrow and we'll see if it's good enough. It's going to be fun. I need to be patient and try to enjoy it. There are so many opportunities to make birdies out there and at the end of the day the best man will win.
'It's a lot of fun and this is what you practice for -- to be in contention on the last day and it feels great to be there. I have very good chance I would say.'
Maritz will also be in contention for his maiden victory on the European Tour and it's almost shocking he's there. Last year, he struggled badly to find his form, but the tall South African has an easy game plan.
'I am just going to enjoy today and then wake up tomorrow and see what happens,' said Maritz. 'I haven't been playing well for the last 18 months so I am just thrilled to be have put two good rounds together.'
David Dixon (68), James Hepworth (67), Jean Hugo (71) and Cesar Monasterio (71) are knotted in seventh place at 6-under-par 207.
Defending champion Joakim Backstrom posted his first round under par in his defense. He shot a 1-under 70 on Saturday and is part of a group tied for 45th place at plus-2.