"If you get defensive or you back off at all against players like you've got this week, then you've got no chance of winning".
"I'm just playing really solid", Oosthuizen told Sky Sports. "I felt like I was just struggling all day, just grinding", said Johnson, who finished with a 2-under 69 and is tied for 13.
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.
Key to his success was an eagle at the par-five 15th hole as he took command of the first of the four WGC events this year. "I was just short of the green and had a very easy chip shot (for eagle)", he said.
"I drove it pretty well today, I've got a lot of confidence with my driver and was just teeing it up on a lot of holes and making good swings".
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 U.S. and impressive Indian Shubankhar Sharma.
Paisley might have expected better after his recent BMW SA Open win, his debut European Tour victory.
TSHWANE OPEN: In Pretoria, South Africa, Louis de Jager's strong finish gave him a one-shot lead after the first round of the Tshwane Open. Starting his day from the 10th tee, Sharma birdied 11 and 12 from 13 and two feet respectively before rolling in a tricky 15-footer for his third birdie on 18.
"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.
In fact, there were just two players from the top 10 in the World Ranking, No. 2 Jon Rahm (seventh) and No. 7 Rickie Fowler (tied for eighth), among the top 10 when play finally concluded in fading light after a 30-minute lightning delay. "I like the course, it kind of suits my strengths". He made six birdies for the round. He and Paisley were one stroke in front of Spain's Rafael Cabrera Bello and Thailand's Kiradech Aphibarnrat, both in the clubhouse on five-under 66.
Johnson, for example, had two bogeys and a double-bogey and played the par 5s, which he feasted on a year ago on his way to victory, in 1 under par.