FANLING, Hong Kong (Reuters) - Tiredness caught up with Rory McIlroy on Thursday and the world golf number one could shoot only a three-over-par 73 on the opening day of the Hong Kong Open.

McIlroy, who this week became only the second player to win the money-list titles in both Europe and the United States, admitted that he was exhausted after a year in which he has won four titles, including the U.S. PGA championship, and helped Europe to win the Ryder Cup.

"I was struggling a little bit with my energy levels; I felt lethargic," the Northern Irishman said after a round in which he produced one birdie.

"I think after clinching the Race to Dubai it was always going to be a little bit of a letdown. I tried hard to lift myself after what happened but that gets harder when you are feeling a bit tired."

McIlroy was not helped by playing late in the day when the Fanling course was affected by a tricky wind swirling through the trees.

Javier Colomo, who plays on the Asian Tour, took the lead with a six-under-par 64.

His fellow Spaniard Miguel Angel Jimenez, who was a member of the European Tour before the 23-year-old McIlroy was born, shot a 65. Should he win his third Hong Kong Open on Sunday at the age of 48 years and 318 days he would become the oldest winner on the European Tour.

The record is held by Irishman Des Smyth who was 48 years and 34 days old when he won the 2001 Madeira Island Open.

Ryder Cup captain Jose Maria Olazabal moved into a share of fourth place with a 66.

Colomo started out by plying his trade on the European Challenge Tour but lost his playing rights so went to the Asian Tour School last year.

The 28-year-old has had three top-10 finishes this year and has earned promotion into bigger tournaments in 2013.

"Now I am 33rd in the Asian rankings and that's perfect because next year I will be playing in all the tournaments co-sanctioned with the European Tour," he said.

(Editing by Clare Fallon)