Assuming equal pilots skills the NIKI will have the edge in that fight, due to its better overall performance.
Even when the A5 is coming in with an E advantage a well flown NIKI will equalize that and than gain the upper hand. As humble mentioned the A5 has a speed advantage down low and in a fight for my life i would use that to return home and come back another day.
As we are here in a game and beside our ego nothing is hurt when we get shot down, you can try to work with that advantage. Try to keep the A5 fast while forcing the NIKI to bleed its energy in hard turns, while you keep the pressure on him using the vertical. If the fight gets slower, either get ready to put the breaks out and avoid a wingline overshot or judge the right moment for an extension.
But even than if the NIKI pilot doesn't go for the "easy" shot, he might just climb out to an altitude where he has an performance edge.
Actually, except for @ 10k, the 190a5 has the speed advantage high or low. Saying that the niki (was another poster) has a "Sweet spot" @ 15k is 100% wrong. 15k is one of the huge drops in the niki's performance between 10 and 20k. 15k would almost be the BEST alt for a 190a5 to fight a niki at (under 20k that is).
The niki is fastest at 10k and @ 19k, with its "best" all around alt to fight at @ 7k ish. (Blend between speed and climb)
Under 15k the niki has the better climb (w/o wep) and 12k and under they have roughly the same climb with wep.
You can beat a niki in a 190a5, but you have to out fly or out think them. A better merge, using the roll of the 190 to setup a vector for a shot, posibly bleeding E at the start to get position for a shot, etc, etc...
If you try to match the niki move for move you will lose in the long run. Don't get stuck in a constant immel/yoyo/turn etc as the niki will steadily gain position. If you are doing the same move more than once/twice you are going to be giving away position. The KEY is to get in position quick and get the shot. The longer the fight goes the more the advantage goes to the niki driver.
I have had several very good a5 drivers make me nervous right at the start of the fight. Heck, last week I had a guy durn near shoot me down in the first 10 seconds. But as I said as the fight progressed I gained position and shot 'em down.
Try to shoot them down in that first 10-15 seconds. Most niki drivers fly with 100% throttle and like dummies don't use the nikis flaps (which drop @ 220kias and are one of it's biggest advantages). You can use that against them and get position early in the fight and hopefully a kill shot. The A5 will bleed E quick and the combo of that and its superior roll should give you a couple decent snap shots early in the fight. If you miss those shots early on, you might consider extending and "reset" the fight. Prolonged stall/knife fights should be avoided.