MEXICO CITY - In a relationship that has spanned nearly 25 years, Jim "Bones" Mackay has rarely missed a day of work as Phil Mickelson's caddie.

His brother's wedding conflicting with a tournament? No problem. Rent a plane, get to the ceremony, return for Mickelson's round the next day. Aching, throbbing knees? Ibuprofen, then offseason double-knee replacement surgery.

Other than a couple of overseas tournaments, Mackay has been there for every meaningful round of golf Mickelson has played dating to his professional debut at Pebble Beach during the 1992 U.S. Open.

And he was present Friday at Club de Golf Chapultepec -- until illness forced him off the course during the second round of the WGC-Mexico Championship.

In stepped Mickelson's brother, Tim, who promptly saw the Hall of Fame golfer bogey the first hole they worked together.

But everything was good from there as Mickelson shot a 3-under 68 to remain in contention, two strokes behind leader Rory McIlroy and in a tie for second place.

"It was the first time we were able to do that, and it was fun," said Tim Mickelson, 39, who until last year was the golf coach at Arizona State and now represents former ASU golfer Jon Rahm for Lagardere Sports -- the same agency that represents Phil.

"Trust me, I don't want Bones' job, though. I have a whole new respect. Every hole seems uphill."

Despite a bogey shortly after losing his regular caddie early in the second round, Phil Mickelson carded a 3-under 68 and is just two shots off the lead at the WGC-Mexico Championship. Buda Mendes/Getty Images

Asked how much advice he gave Phil, Tim showed a touch of his older brother's sense of humor: "Well, he's never listened to me on or off the course, so nothing is going to change."

And he didn't expect to be paid, either. "Dinner would be nice," he said.

Mackay had not been feeling well due to a stomach ailment and said he would try to make it. But carrying a 40-pound golf bag on a hilly course in thin air eventually took its toll. Phil had called his brother, who was eating breakfast at his hotel, and asked if he could be on standby just in case.

Tim got to the course and followed for the first few holes (Mickelson started on the back nine). But during the par-3 13th, Mackay said he could no longer continue. Tim took over from there.

"You can't replace somebody like Bones," Phil said. "He's phenomenal, and he's so good with club selection and strategy and so forth. But my brother's a great player, and he did a phenomenal job today. He's caddied in amateur events, and he's played at a high level in amateurs, so he has a good grasp of this."

Tim Mickelson, shown with his big brother at an Arizona State football game in 2013, was a reliable backup caddie for Phil on Friday. AP Photo/Matt York

But not enough to get too heavily involved. Tim is a scratch golfer who could occasionally hold his own against his big brother when he played more often.

And given how often they've played golf together over the years, Tim figures to have a pretty good grasp on Phil's game.

"Yeah, but not tournament golf," Tim said. "When he and I go play, he can do whatever he wants, and it's fun to see, but tournament golf is quite a bit different. I was pretty much a bystander and carried the bag and just tried to not get in anyone's way."

The two conferred on yardages, but when it came time to pick a club or read a putt, it was all Phil. And that is where a really good caddie who is confident in his abilities -- and his relationship -- might step in to offer another opinion.

Tim didn't do that. But was he tempted?

Jim "Bones" Mackay, Mickelson's longtime caddie, was fine during practice in Mexico City earlier in the week, but he couldn't get through Friday's second round because of a stomach ailment. Jamie Squire/Getty Images

"I'm not going to answer that," he said. "That's going to come back and bite me some day."

The 7,400-yard, par-71 course at nearly 7,800 feet of altitude has presented all manner of challenges to the players in the WGC field.

But because the greens are so tricky, Phil Mickelson said he would not have been calling Mackay in to help him with putts. And because the altitude adjustment makes choosing a club as much about feel as distance, there would be less interaction in that regard, too.

"For the most part, it's very difficult for a caddie to pull a club for a player here," Mickelson said. "So I've got to do it because it's based on how I'm feeling, how hard I want to hit it, what kind of draw or fade I want to hit, because the discrepancy in distance is so great here that it's hard for any caddie to pull a club. So I don't use Bones all that much on the club selection, either, even though he'll give me a few options. So of all places for it to happen, this is probably the easiest one.

"I'm least reliant on him here, whereas at Augusta (where Mickelson has won the Masters three times), it would be catastrophic because we have so much history on that golf course over the years."

Mackay has caddied for Mickelson in 41 of his 42 PGA Tour victories, including all five major titles. The lone tournament title he missed came in 1991 when Mickelson was still an amateur and the two had yet to meet. Caddying for Mickelson at the Tucson Open was Steve Loy, then Mickelson's coach at Arizona State.

Loy would later leave that job to become Mickelson's agent, a role he still holds today.

"I got fired before he turned pro," Loy joked about his brief tenure as Mickelson's caddie. "What kind of deal is this? I thought I did well. But he went on to win 41 more."

Mickelson would dearly love to win another one, with Mackay on the bag, of course. That was still to be determined after the longtime caddie visited with a doctor before retreating to his hotel room. His prognosis for the rest of the weekend was unclear.

But Bones surely knows what Tim Mickelson saw on Friday.

"The coolest thing is that after he's been out here 24, 25 years, to see how competitive he still is," Tim Mickelson said. "You know, 'I want one more, let's get one more.' That fire is definitely still there."