Sharma intends to keep his foot on the pedal despite being in his first WGC tournament which is featuring 45 of the world's top-five players this week.
South Africa's Oosthuizen was blemish free in his 64 to be one ahead of three players - Chris Paisley of England, Xander Schauffele of the USA and impressive Indian Shubankhar Sharma. (It was) a bit of a scrappy finish with a few missed clubs and a poor shot on 17 but I held it together.
"I'm very confident with my driver at the moment".
However he stumbled at the 10th and 17th to offset gains at 11 and 15 to finish even for the nine and likely pleased enough with his play.
"After the win in SA, I didn't want to just be happy with winning".
"I was delighted with the score, and obviously it's the biggest tournament I've played in by a long stretch", said Paisley, who finished first, fifth and fifth in his first three starts on the European Tour this year.
Right behind were two players unfamiliar to the world stage, though they sure didn't play like it.
"It's probably the worst I've ever felt over the ball in my life", Thomas said.
Victor of the 2010 Open Championship, South African Oosthuizen opened with a seven-under-par 64 in Mexico on Thursday.
In case you missed the Mickelson magical mystery tour in Mexico previous year - or you just want to see them again (you should) - the PGA Tour has put together a wildly entertaining compilation of some of his best escapes from the week. "I really kind of started on Tuesday, wasn't swinging it very well, and then yesterday I just was hitting it terrible".
Schauffele had six birdies in his 65.
Thomas, who is fresh off his eighth Tour victory last week at the Honda Classic, couldn't say the same thing.
He played with Johnson and Jon Rahm, who also had enough mistakes to limit him to a 67. The 24-year-old, who ranks 164th in putting strokes gained in 2018, cost himself 1.55 strokes on the green in the first round at Club de Golf Chapultepec.
Oosthuizen took advantage and moved ahead by holing a five-foot eagle at the 15th, with Schauffele birdieing two of his last four holes to close a blemish-free card and join Paisley on six under.
"At times it has been a great struggle because I know I can win out here, but I just never have been given that chance yet", Song said.