McIlroy slumps at wintry Wentworth
With some players wearing beanies and thick mittens for protection against chilly eight degree temperatures and winds gusting up to 20mph, it was Finn Mikko Ilonen who looked most at home as he grabbed the early clubhouse lead on five-under 67.
McIlroy, still waiting for his first victory of 2013 after winning the orders of merit on both sides of the Atlantic last year, limped home in 41 strokes at the European Tour's flagship event after bogeying five of the last six holes.
The twice major winner capped a disappointing round with bogey sixes at the 17th and 18th.
Playing partner and close friend Graeme McDowell, who landed the World Match Play title in Bulgaria last week, also carded a 74 and fared even worse than McIlroy at the last hole as he ran up a double-bogey seven.
Ilonen, though, sprinted through the pack at the end of his round by picking up birdies at the 14th, 15th and 18th.
"I stayed patient all the way today," the double European Tour winner told reporters. "It was pretty nippy this morning and we had some hail as well.
"All day I was thinking, 'Can I get these waterproof trousers off'?...but I couldn't."
Ilonen, who lifted the Indonesian Open and Scandinavian Masters titles in 2007, has a good chance of staying in front all day because the forecast is for the weather to get even worse later in the round.
"I said to my caddie when we started that this was a good draw because you can get a 3.30 tee time and it won't be the best time to play," said the 33-year-old.
"I said to a friend of mine yesterday, 'I think it's going to snow tomorrow' and she started laughing. This is the British summer and we have proof of that."