Steve Darcis ousted former champion and fourth-seeded Radek Stepanek in the second round of the SAP Open, winning 7-6 (7), 7-6 (7) on Thursday.
Stepanek of the Czech Republic is ranked 29th in the world while Darcis is ranked 72nd and is 22nd-seed of the tournament.
Darcis' opponent in the quarterfinals will be fifth-seeded Julien Benneteau of France, who defeated Gilles Muller of Luxembourg, 6-2, 6-4.
South Africa's Kevin Anderson also advanced to the last eight, benefitting from two day's rest against a tired Matthew Ebden. The Australian had played until 2:30 a.m. local time on Wednesday to advance to the second round.
Qualifier Dimitar Kutrovsky of Bulgaria beat American Blake Strode, 7-5, 6-3 to also advance to the quarterfinals.
Strode, playing in his first ATP main draw match, was a late substitution for defending champion Gael Monfils who withdrew from the tournament with an inflamed tendon on Wednesday.
The US-born Strode is ranked 393rd in the world, but still higher ranked than No. 435 Kutrovsky, who will be an unlikely quarterfinalist against the winner of the later Robby Ginepri-Ryan Harrison match.
And the winner of the Michael Russell-Denis Istomin match will square off against second-seeded and tournament favorite Andy Roddick in the quarterfinals.
Roddick is playing with a slight tear in his right hamstring sustained during the Australian Open. He also twisted his right ankle during his match against Denis Kurda.