Mickelson found himself having to call on his brother to carry the clubs after regular caddy Jim "Bones" Mackay also found himself battling with food poisoning.
There has been no shortage of entertaining and incredible moments this week at the WGC-Mexico Championship - from Phil doing the most Phil things we've seen this year to Dustin Johnson hitting 322-yard 2 iron shots.
Mcilroy holed from the 14th fairway for eagle on his way to a six-under 65 to build a two-shot lead.
After producing a birdie at the 15th, McIlroy was three clear, a status reduced only because of missed putts from inside 4ft on the last two holes.
McIlroy, whose only other appearance this year came at the South African Open in January, carded six birdies and one eagle during Friday's session at Chapultepec Golf Club. "I think the more rounds you play out here, the more you can kind of get dialed into trusting your numbers in this altitude".
And the four-time major victor was ideally placed to fulfil his part of the bargain after a second round of 65 at Chapultepec Golf Club which left him two shots ahead of Phil Mickelson, Justin Thomas and England's Ross Fisher.
Fortunately for Mickelson, his brother Tim had been alerted to the problem and was on hand to take over on the bag as his sibling looked to remain in contention for a third World Golf Championship title. "He is one of the best in the business". "Hit it a little straighter today and then really just stayed aggressive".
And when he also bogeyed the third, the four-time major victor found himself a shot behind Johnson and Mickelson, who had chipped in for a birdie on the first and picked up another shot after chipping in on the fourth. "My body is in great condition and if I keep playing well, I have a chance going into the weekend".