Charl Schwartzel fired a flawless seven-under-par 64 to take a share of the Madrid Masters lead with Australian Marcus Fraser, but revealed afterwards he was struggling with injury and illness and had considered withdrawing.
The South African was tied on nine under with Fraser, who carded a five-under-par 66, at the end of day two, giving the pair a two-shot lead over Swede Steven Jeppesen and Englishman Robert Rock, who posted 67 and 65 respectively.
Five players are three shots back on six under, including Spaniard Santiago Luna (68) and Italian Paolo Terreni (65), who has made only one cut in 19 previous attempts on the European Tour.
The major news from Madrid however, was on the other side of the city in La Paz hospital where it is reported doctors have discovered Spanish legend Seve Ballesteros is gravely ill.
The five-time major winner fainted on Monday and has remained in intensive care while tests have been carried out. On his website he claimed to be feeling fine and thanked well-wishers for their support, but a statement confirming a serious illness is expected at the weekend.
Back on the course though, Schwartzel, out early off the 10th tee, had a run of three birdies from the 13th and then increased the pressure on the way back in with an eagle and two more birdies.
He briefly led at one point, capitalising on a slip-up from Fraser, who had a three-hole run of bogeys following a strong start but recovered by picking up five shots on the back nine.
However, afterwards Schwartzel said he was just glad to have completed his round, let alone be a co-leader.
"It's funny because I was going to withdraw this morning but I spoke to my dad and he told me to tee off and see how I feel," he said.
"I've had a sore shoulder the last couple of days and I've been sick for a while. I'm taking antibiotics and I don't feel that strong."
"I really didn't feel good this morning but I got a couple of birdies early on and felt a bit better. Then the sun came out and my shoulder warmed up and improved a lot."
He added: "It was a really solid round. I played well and felt very comfortable."
"My swing's been feeling good for a while now and this golf course suits my game. I hit a lot of good shots today."
Fraser, the 2003 Russian Open champion, set the pace early on with a glut of birdies, one of which came from a chip-in at the third.
Afterwards he said his in-form short game was a valuable asset on the Club de Campo course.
"I like the course," he said. "It's a bit shorter than other weeks, not so much of a slog-fest, which suits me well because it tests your short game."
"I feel like I'm playing quite well overall."
Englishman Paul Broadhurst, who was a joint leader overnight, had a 72 to slip back to three under overall, but moving in the other direction was another Australian in Andrew Tampion, whose 63 was the round of the day and propelled him to four under.
Tampion is one stroke ahead of New Zealander Steve Alker, who posted a five-under 71 second round to be at three-under overall and in a share of 21st place.
Further down the leaderboard, Aussie Peter O'Malley managed an