Three months after winning that first event, Sharma is on the verge of another major breakthrough, holding the 54-hole lead in his first PGA event.
England's Tyrrell Hatton and Spaniard Rafa Cabrera Bello both shot 67s to tie for third at 15-under 269.
Mickelson though, kept his cool and responded with birdies on the par-5 15th and the 16th to draw level with Thomas. Entering the WGC-Mexico Championship, Sharma is No. 75 in the official World Golf Rankings.
"I can't put into words how much this means to me, it's been a long time", said Mickelson. The youngest player at the tournament will be paired with one of the oldest, Phil Mickelson, in the final round on Sunday.
Phil Mickelson ended a five-year run without a title on Sunday with a playoff victory over Justin Thomas in the WGC - Mexico Championship. "Otherwise I just got the ball in play all day and it was great to finish with three birdies in a row", said Sharma. The best I think was when Rory won the US Open.
Justin Thomas of the U.S. produced the round of the day, carding a course record 62 to cut the deficit to Sharma to four strokes in a tournament that is one of the four annual World Golf Championships organised by the International Federation of PGA Tours. The victory in the Joburg Open helped Sharma earn his European Tour pro card, allowing him to compete in more high profile events in the future. I was telling to Baaz, who's on my bag there, watching all of these guys on the range, it just seems like there's a TV in front of me and I'm watching them through the TV.
"It was always a matter of when, not if [he would win again]", said Mickelson's brother, Tim, who replaced Jim "Bones" MacKay on Lefty's bag a year ago.
Sharma was six when he went to a golf course for the first time and got hooked on the sport.
Sharma survived some anxious moments to maintain his two-shot lead ahead of some of the biggest names such as Sergio Garcia, Dustin Johnson and Jordan Spieth, apart from Mickelson.
"I'm definitely disappointed", he said. "I haven't really watched him play".
"To me, he has a very calm attitude". I just wanted the first round to get out of the way.
Sharma has gone about his business in Mexico so quietly that when he sheepishly approached Mickelson, a 42-time PGA TOUR victor, at the practice putting green to introduce himself prior to his third round, the World Golf Hall of Famer initially waved him away thinking he was a journalist.