Johnson produced a masterful ride in giving the 8yr-old the most economical run he could and it paid off when Dez got his nose down right on the line to beat Rosewood in the 1600m feature.
It was not just Johnson's efforts that helped him win the $100,000 event.
The rider could have picked a number of the best Winter Cup contenders, but he stuck with Dez, who had been out of form until his last-start win at Otaki.
Johnson revealed after the race that the horse's big run for third in last year's Winter Cup was part of why he stuck with the Simon Wilson-trained galloper.
"He's had a few issues in this prep, but I had a look at his run last year, which was huge, and on his last start it looked like he was back in that form," he said.
Runner-up and favourite Rosewood hit the lead early in the run home, before Johnson unleashed Dez from his cosy run in transit.
Dez levelled up to Rosewood just short of the post but Johnson was not certain he had timed his run to perfection.
"He travelled nice in the race, although when we moved into second halfway down, he was flat out," Johnson said.
"The second horse kept kicking and [Dez] just lifted that last bit.
"I wasn't sure if we had got up, as I've had a run of seconds and thought I was second again."
Dez's victory gave Johnson his first Winter Cup victory since his win with Robyn's Affair in 1991.
Wilson had put Dez's poor start to his winter campaign down to choosing the wrong races for him.
Dez put three mediocre runs behind him with a good win in a fair rating 82 contest at Otaki last weekend.
The win booked the Zed galloper's ticket to Riccarton and once the horse got to the South Island, Wilson's confidence levels rose.
"We've had a few problems, but we sorted them out and he has definitely hit top form at the right time," the trainer said.
"I was happy with him, as he had travelled well and was eating up, but you just never know on the day.
"Being by Zed, we knew he would handle the conditions and he had a great rider on board."
Fourth-placed Red Sierra produced one of the runs of the race and a performance that suggested he will be tough to beat in this Saturday's Winter Classic.
The David and Emma Howarth-trained 8yr-old was very slow to begin for rider Samantha Collett.
Red Sierra tailed the field by six lengths at the 1200m, before producing a huge finish.
