Charl Schwartzel secured a one-stroke advantage after shooting a 66 on the penultimate day at the Madrid Masters.
The South African moved to 14 under after five birdies and an eagle, with Argentine Ricardo Gonzalez lying second following a blistering inward nine.
Gonzalez came home in 27, a European Tour record this season, with six birdies and an eagle, to post a 62.
Miguel Angel Jimenez lies 13 shots off the lead after a 72, while Jose Maria Olazabal's 71 put him one further back.
Victory for Jimenez would take him to third in the Order of Merit, though there is very little chance of that now.
The colourful Spaniard went out in 39 after a double bogey at the fifth, although he managed two birdies on the back nine.
Olazabal, in his first event since June following injuries and illness, had three birdies but bogeyed three of the closing eight holes.
The stunning finish from Gonzalez might well have given him the outright lead, but Schwartzel holed a tricky putt for par on the 17th and then holed from 10 feet to birdie the last.
On a day of impressive scoring, Spaniard Pablo Larrazabal also posted a 62 to claim sole possession of third place on 11 under.
Gonzalez, 38, had the last of his three European Tour wins in Spain in 2004 and has not finished higher than joint 19th in six previous events on the Tour this year.
He admitted his round was a surprise and said: "I was one under after nine and thought to myself 'I need a few more birdies' - but to shoot nine under is unbelievable.
"The key, I think, was my drive on 10. From there, I felt more confident. My putting was also very good on the last seven or eight holes.
"Spain is like a second home, and I feel very good here. I'll feel a little bit of pressure in the last round but I'll try to make a good score again and play well.
"I will just play golf and enjoy it - that's the only thing I can do."
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Schwartzel, the joint halfway leader with Australian Marcus Fraser, was full of admiration for Gonzalez's performance, and looked forward to the final pairing on Sunday.
"Sixty-two is a great score, and we'll have a good battle," said the South African, who has recovered from a shoulder problem but is still feeling the effects of illness.
"I will try to do the same as the last three days and stick to my game plan. I'm hitting the ball well and I'm confident in my swing."
Fraser, however, had five dropped shots in a 74 and slipped into a share of 14th eight shots adrift.
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