Last year Shirley, 49, used PE’s wind to his advantage when he won his fourth successive title at the event.
Despite not feeling his best, Shirley said he would still go out and “retain my trophy”.
“I’m still standing and breathing,” he told In the Bunch today. “If I can do that I can still ride a bicycle.
“This will be my fifth title I’ll be going for and I’m not going to give it away easily. I’m probably going to have a fitness test today or tomorrow to see where I am health- and fitness-wise.
“If I’m not healthy on the day I will still go along anyway and see what I can manage. If they’re going to beat me they’re going to have to try hard.”
Shirley said his flu had been “lagging on for a while”, adding that it had hampered him during important parts of the season.
“It happened over the days when I needed to do the bigger stuff and climbing, so I’ve been a bit short. But I have hundreds of thousands of kilometres in my legs.
“Hopefully that will pay off,” he added.
The Johannesburg-born rider finished second overall last year, losing out to former SA U23 road champion Clinton Barrow. But due to the latter having not been a part of the PE Masters & Ladies Club, Shirley kept the trophy.
He said he knew from previous events that he had a penchant for racing in adverse weather conditions.
“My skill is to ride in really bad weather,” Shirley said. “The windier it is, the better for me. That’s why I got a good result last year, because it was blowing a gale-force westerly.
“So [last year] we basically dropped everyone going up towards Van Stadens. This year it looks like it could be wet and not as windy, so I’ll just have to play it by ear, also depending on my [health] situation.”
He added that if a westerly blew again the race situation would be about “getting rid of the weaker riders” by the time they turned for the finish line.
However he said the standard of cyclists in PE had “definitely increased”.
“We’ve got many new elite youngsters. I think Bradley Gouveris will be present.
“It’s not a race that you can take lightly, due to all the climbing. If you’re going to break away and the wind is blowing [from the] west then you’re going to have a long day out there.”
Shirley said he was confident as it was his race to win.
“If I have a good day then the others will have to fight hard to take the trophy from me. I’m confident in my ability.”