'What amazes me is that I have had three weeks off and not hit a ball and have come out and played really well,' said Mackenzie, who recently returned from a vacation in Florida. 'It comes to show it is not all down to practice and hard work sometimes. Sometimes a layoff does you good.'
Mackenzie was five shots off the lead to start the day but made an early surge with a pair of birdies over the first three holes at Le Golf National. A poor tee shot led to just his fourth bogey of the week, but he put the mistake behind him with a birdie at the eighth hole and a long chip-in eagle at the par-five ninth.
Three more birdies on the inward nine allowed Mackenzie to top a leaderboard heading into the final round for the first time since the 1990 Murphy's Cup. He wound up finishing second in that event, his best showing since he joined the European Tour in 1981.
'I haven't won in all those years and you keep hoping. I don't give up. You just hope your day will come,' he said. 'But you need to be there after three rounds to have a chance and I have a chance now. Hopefully I will play tomorrow like I did today. But I am not going to get too excited because I know what this game is like.'
Mackenzie did win the Zimbabwe Open in 1985. Among his 33 career top-10 finishes on the European Tour was a tie for fifth in the 1992 British Open at Muirfield and a 10th-place showing in the 1994 Volvo PGA Championship at Wentworth.
The British journeyman will have to fight off a challenge from Olazabal, one of the hottest players on the planet with two victories and seven top-five finishes worldwide in the last five months.
The two-time Masters champion from Spain got off a sensational start Saturday when used a four-iron to ace the 202-yard, par-three second, a hole he bogeyed on Friday and double-bogeyed in the opening round.
After adding a birdie at the third hole, Olazabal reeled off nine straight pars but finally broke the chain with a birdie set up by an approach inside four feet at the 13th. He dropped a shot at the par-three 16th before closing with two birdies for his second straight round of five-under 67.
'It will be interesting tomorrow,' said Olazabal, who captured this event a year ago at Lyon Golf Club. 'There is a big pack of players who can win the tournament, anyone within five or six shots of the lead. It will be tough but at least I am giving myself a chance.'
Immelman, who jumped out to a four-shot lead with scores of 68-64 over the first two days, came back to the pack after a one-under 71 made up of four birdies and three bogeys.
'I felt I played nicely but just couldn't get going,' said the 22-year-old South African. 'It was just one of those days. I felt I hit some good shots and they didn't quite work out and made some good saves other times.
'All in all it was a pretty good day. I haven't shot myself in the foot and I am right where I want to be. I am within striking distance.'
Denmark's Anders Hansen matched Mackenzie for low round of the day with a 65 to finish alone in fourth place at 12-under. England's Gary Evans (67) and France's Jean-Francois Remesy (69) were next at minus-11.
Remesy is looking to become the first Frenchman to win the Open de France since Jean Garaialde in 1969.
Ian Woosnam of Wales carded eight birdies in a 66 and was four shots back.