Mark Foster and Gary Murphy are knotted in second place at 4-under-par 66.
Spain's Miguel Angel Jimenez, a two-time winner on the 2005 European Tour schedule, posted a 3-under-par 67. He is tied for fourth place with Abama Open winner John Bickerton, Open de Portugal champion Paul Broadhurst, Sam Little and Simon Kahn.
Lomas began his opening round on the back nine at Pula Golf Club and collected his first birdie at the par-4 11th. He parred the next three and birdied 15 and 17 to make the turn at 3-under 32.
On the second nine, Lomas, whose only European Tour victory came at the 1996 Czech Open, and did not move up the leaderboard with his early play. Lomas parred his first five, then birdied the par-5 sixth.
Lomas was knotted in the lead at minus-4, but a birdie at the ninth propelled him into first.
Foster recorded all of his birdies on Thursday in a six-hole span. He tallied birdies at five and six, then parred Nos. 7 and 8. The Englishman birdied nine and 10 to get within one of the lead.
Murphy flew out of the gate with a birdie at No. 10, his first on Thursday. The rest of his opening nine was inconsistent as he mixed two birdies and two bogeys.
The Irishman posted birdies at four, five and nine for his share of second place.
Jesus Maria Arruti, Mattias Eliasson, Klas Eriksson, Gonzalo Fernandez- Castano, Fredrik Henge, Jose Manuel Lara, Jose-Filipe Lima, Marten Olander and David Park share ninth place at minus-2.
Defending champion Sergio Garcia opened with a 1-under-par 69 and is tied for 18th place.
'I'm satisfied because this course is playing tougher than last year,' said Garcia, ranked sixth in the world. 'I felt like I struck the ball nicely. It was just one of those days where I couldn't get anything going.'
European Ryder Cup captain Ian Woosnam withdrew from his final scheduled tournament on Thursday. He hurt his shoulder and pulled out after hitting out of the rough on 17.
'It's a disappointing way to finish the season,' said Woosnam, who took fifth last week at the Open de Madrid. 'After last week I was looking for a good tournament. I hurt my shoulder on the fifth tee - just hitting it too hard when you're too old.'