Chardy eliminated sixth-seeded Juan Martin del Potro 6-3, 6-3, 6-7 (3-7). Del Potro is the first Top 10 player on the men's side to lose as the tournament reaches the Round of 16.
Chardy fought off four break points in building 2-0 set lead before del Potro started his comeback. His only breaks of the match -- one in each the third and fourth sets -- allowed him to pull even at 2-2 but Chardy had a break in the fifth game of the third set and held on from there for the win in 3 hours, 45 minutes.
Seppi, the 21st seed, ousted No. 12 Marin Cilic 6-7 (2-7), 6-3, 2-6, 6-4, 6-2 with five of his nine breaks coming in the last two sets. Their match lasted more than 3 1/2 hours.
The fourth round ended early Sunday in Melbourne as 14th-seeded Gilles Simon escaped with a 6-4, 6-4, 4-6, 1-6, 8-6 decision over Gael Monfils. Simon had a break to get to 5-4 of the fifth set but Monfils broke back -- his ninth of the match -- to even the score.
Simon, though, posted a 13th game break and held from there to win the match in 4 hours, 43 minutes. The final set took 94 minutes.
Second-seeded Roger Federer and No. 3 Andy Murray each advanced to the fourth round with straight-set wins. Federer topped local favorite Bernard Tomic 6-4, 7-6 (7-5), 6-1. Federer faced one break point in the match and only four so far in the tournament.
Murray beat Ricardas Berankis 6-3, 6-4, 7-5 despite dropping serve once in each set. Murray, however, had two breaks of his own in each set.
Also Saturday, seventh-seeded Jo-Wilfried Tsonga stopped Blaz Kavcic 6-2, 6-1, 6-4; No. 9-seeded Richard Gasquet got by Ivan Dodig 6-4, 6-3, 7-6 (7-2), 6-0; and 13th-seeded Milos Raonic defeated 17th-seeded Philipp Kohlschreiber 7-6 (7-4), 6-3, 6-4.
Federer draws Raonic while Murray is to play Simon in the fourth round. Chardy and Seppi also play in Murray's quarter of the draw with Tsonga and Gasquet squaring off in Federer's section.
Swim Week Miami Beach 2014 [PHOTOS]