Damien McGrane, who held the lead after the opening round, was still on the course when the round was called and stood at 5 under par after a bogey at the 18th, his ninth. Maarten Lafeber, who won last week's Dutch Open, was also at minus-5 through 13 holes.
Park jumped out of the gate on Friday with birdies on each of the first three holes at Pula Golf Club. He added a birdie at the par-5 sixth to make the turn at minus-4 and continued his fine play on the back nine with a birdie at the 12th.
The Welshman, whose lone European Tour victory came at the European Grand Prix in 1999, bogeyed the 13th but responded with birdies at the 15th and the 16th to take the clubhouse lead at 6-under-par 134.
'I played well tee to green today and my putting was a lot better today than it was yesterday,' said Park. 'It's been four years since I last won a tournament and obviously that's not what I've aspired to but you've got to take your chances while you can.'
Benn Barham and Henrik Stenson were in the clubhouse at 4-under-par 136 after matching rounds of 68.
Among those still on the course at minus-4 was local favorite and two-time Masters champion Jose Maria Olazabal.
'I have to say that I think they suspended play at just the right time,' said Olazabal. 'Everyone was able to get back into the clubhouse before it got very, very wet.'
Play is scheduled to resume Saturday at 8:45 AM local time.