Also, Australian Bernard Tomic beat Lukas Lacko of Slovakia, 6-4, 6-2.
"Those break points (at 1-2) were important because I felt like if he had have got that early and been up a break 3-1 it's a whole different story", Tomic told Tennis Atlantic. "I played pretty well and the courts and conditions suit me".
"Those matches in Kooyong kind of don't count", Tomic said.
With the 2019 monkey off his back, Tomic is desperate for more court time as he seeks to regain match fitness.
"We've played a couple of times".
"He has one of the best serves in the world".
"But I have to go out there and try and play the way I did today".
Sandgren, the eighth seed, was edged by Paolo Lorenzi 7-6 (7-4) 6-7 (4-7) 6-4, while Jordan Thompson got through, beating Noah Rubin 6-7 (7-9) 7-5 6-1. He has primarily competed on the ATP Challenger Tour over the past six months, but qualified for the main draw this year at ATP Tour events in Doha and Cordoba.
The Australian served nine aces in a 54-minute victory - his first of the year - and next faces top seed John Isner.
At the Argentina Open, the only seed in action departed, Nicolas Jarry going down to qualifier Lorenzo Sonego 6-7 (3-7) 6-4 6-3.
The qualifying rounds were completed with Tomljanovic, Blinkova, Karolina Muchova and Lin Zhu wining through while Alison Riske and Hercog grabbed the lucky loser spots.