MEXICO CITY -- Why did Phil Mickelson get a free drop on Saturday when his ball could not be found after a wayward tee shot at the 10th hole?

Mickelson hit his drive well left of the fairway at Club de Golf Chapultepec during the third round of the WGC-Mexico Championship. When arriving at the area where he believed the ball to be, Mickelson and caddie Jim Mackay could not find it, but they were told a spectator picked it up.

It was the first of three straight holes on which Mickelson received rulings and ended up getting free drops, meaning no penalties. The resolution fell under Rule 18 of the Rules of Golf that deals with if ball at rest moved is by an outside agency.

Phil Mickelson applied the Rules of Golf in Round 3 of the WGC-Mexico Championship and it kept his chances of victory alive for the first time since The Open at Muirfield in 2013. Buda Mendes/Getty Images

The situation at the 10th hole turned out to be the most important because a lost ball means a stroke and distance penalty. Mickelson would have had to return to the tee to hit his third shot and would have been looking at making a bogey or double-bogey.

Instead, the drop led to a par as Mickelson scrambled his way to a 3-under-par 68 in Round 3.

Mickelson summoned a rules official and the PGA Tour's Stephen Cox got to an area left of the fairway that was actually one of the forward tee boxes for the 18th hole.

"There were probably 50 to 100 people around the tee,'' Cox said Sunday prior to the final round of the tournament. "Through the testimony of a number of spectators, it was clear that the ball had been picked up from the teeing ground. It had been spotted by a number of people. From my standpoint, it was relatively straightforward.''

Cox figured that a spectator among many using a crosswalk in the area saw the ball in an unlikely spot and picked it up.

"Once we determined exactly where it was on the teeing ground, [Mickelson] dropped and the ball rolled close to the hole, twice,'' Cox said. "So he was allowed to place it.''

From there, Mickelson had 136 yards to the hole, hit his second shot on the green and missed a birdie putt from 12 feet, settling for par.

On each of the next two holes, Mickelson hit tee shots well right, both times leading to rulings that involved a sprinkler head. He got a free drop on both occasions, making a par at the 11th and a bogey at the 12th.

"I just lost it there for a nine-hole stretch, and that was very disappointing,'' Mickelson said Saturday afternoon.

He entered the final round 2 shots back of 54-hole leader Justin Thomas.