After five days at the year's first grand slam without any major shocks, two came along within minutes as Jeremy Chardy downed Argentine sixth seed Del Potro and Andreas Seppi showed 12th seed Marin Cilic the door at Melbourne Park.
Del Potro, the 2009 U.S. Open champion, had battled back from two sets down to level the contest but the mercurial Chardy grabbed a break in the decider and held his nerve to serve out for a 6-3 6-3 6-7 3-6 6-3 win.
"I had nothing to lose today so it was easy to play," said the world number 36. "I knew that if I wanted to win the match, I had to go for it from the start.
"It's a big win for me, maybe the best of my career."
As Chardy was packing away his rackets on Hisense Arena, Italian Seppi was securing his place as the Frenchman's opponent by wrapping up a 6-7 6-3 2-6 6-4 6-2 win over Croatian Cilic, a semi-finalist in 2010.
In Image: Argentina's Juan Martin Del Potro reacts during his third round loss to France's Jeremy Chardy