A birdie at the last ensured Rory McIlroy maintained his lead at the halfway point of the Dubai Desert Classic, but he was far from his fluent best on Friday.
McIlroy eventually signed for a 70 that saw him reach 11 under par, one stroke clear of American Brooks Koepka in second who came charging through the field with an excellent 65.
McIlroy, who admitted afterwards that he was suffering with an upset stomach, struggled from the tee in particular and, having at one point looked like building a substantial lead, was ultimately grateful to finish out in front.
But he still had a considerably better day than playing partner Tiger Woods who could get nothing going and signed for a 73 that left him eight shots off the pace.
Danny Willett matched Kopeka's 65 to surge into a share of third at eight-under alongside Julien Quesne (70) and Damien McGrane (70), but McIlroy's biggest danger looks to be Henrik Stenson who fired a 67 to climb into joint-sixth at seven-under.
The Swede has been joined at that mark by a host of other players including defending champion Stephen Gallacher, England's Robert Rock and Jamie Donaldson of Wales.
McIlroy dropped his first shot of the tournament on his opening hole on Friday and also hit bad drives at the next two holes, although he did go on to birdie the par-five third and, after picking up further shots at the fifth and seventh, looked set to pull clear on the easier back nine.
But after a birdie at ten, he three-putted from 15 feet for bogey at the par-five 13th - the second from no more than 18 inches - before another poor drive saw him surrender a further shot at 16.
He bounced back to birdie the 18th after putting his third close with a wedge and remained bullish about his chances despite an up-and-down round.
Asked if he still expected to go on and win the tournament, he told Sky Sports: "Yeah, I do. I'm leading the golf tournament, I've been in this position before and I've gone on to win.
"I've led from the front quite well in the tournaments I have won, I know the course just as well as anyone else so I expect to go out there and shoot two good scores and hopefully lift the trophy."
When quizzed about his missed putt from a matter of inches at 13, he added: "It really turned a lot past the hole so I was sort of hitting it inside-left thinking the green was going to take it into the middle of the hole and it just stayed there and I had a hard lip-out.
"But I regrouped okay after that... I'll go back tonight and regroup and try to realise I'm still leading the tournament."
For his part, Stenson did not drop a shot during a consistent second round that continued his excellent recovery from being three over par after five holes on Thursday.
"I think there is still a little way to go but today was the best I have putted for a long, long time," stated the FedEx Cup and Race to Dubai champion.
"I was threatening on every putt and kept the score together with no bogeys, which is always a good sign."
Who will perform better at the Dubai Desert Classic?