"Marcos is an incredible guy. I can remember watching him in the final of the Australian Open at one or two in the morning. I have watched him since I was 13 so it was an unbelievable morning for me and then to come out and play Kevin...it's been quite a day," he said.
"There are so many ups and downs but to beat those two guys on the same day is definitely going to be pretty special. I will remember that one for a while," said Johnson.
His win over the South African world number 20 was the second big upset of the day after compatriot Jack Sock dispatched second seed Tommy Haas in straight sets.
Haas struggled to find his range in his first competitive outing of the year but he wasn't really allowed to by Sock, who controlled the match.
The 21-year-old American, who was a wildcard entry to the tournament, set the tone with his first serve, firing down eight aces and winning 80 per cent of his first serves.
"Some of the important points I didn't play well, that's for sure," Haas said. "You can't get much of a rhythm when he's serving pretty well or you don't allow yourself to get the return back in play. The quickness, the little steps are definitely not there yet. It was a little bit of a struggle, no question about it. He played well when it counted."
Frenchman Benoit Paire was the third seed to fall on day three of the tournament this afternoon, slumping to a 6-3 6-4 loss to Roberto Bautista Agut of Spain - a player ranked nearly 50 places below him.
German Philipp Kohlschreiber advanced through to the third round with a straight forward 6-4 6-2 win over Argentina's Horacio Zeballos.
After a tricky first-round assignment against Spaniard Pablo Carreno Busta, Kohlschreiber was pleased with his clinical display today.
"Today was a new day. I played a great match and didn't make many mistakes. I served well, very clever with the wind. It's tricky in the wind. Overall I played a very clever and great game," said Kohlschreiber.
Kohlschreiber will meet John Isner in the third round after the big-serving American edged past Slovakia's Lukas Lacko this afternoon. Isner smashed a total of 21 aces - clocking 231km mid-way through the third set - as he overcame a slow start to defeat Lacko 7-6(1) 3-6 6-2.