Seppi, the No. 2 seed, was up 6-1, 4-0 when Andreas Haider-Maurer retired due to injury. Seppi had piled up five breaks in six receiving games when the match was stopped.
Robredo rolled past Viktor Troicki 6-0, 6-4, losing seven points in his eight games serving. He backed up his 82 percent success rate serving by winning nearly half (29-of-60) of the points off Troicki's serve.
Aljaz Bedene routed fourth-seeded Alexandr Dolgopolov 6-3, 6-1. Bedene took three-quarters of the points when serving and also ended up with five breaks.
Horacio Zeballos advanced at the expense of No. 8-seeded Carlos Berlocq with a 3-6, 6-3, 6-2 win. Zeballos was 3-for-3 in break chances in the third set.
The quarterfinals Friday have Robredo going against Bedene while Seppi draws Zeballos. The top half of the draw sees Albert Montanes playing Gael Monfils and third-seeded Fabio Fognini meeting No. 6-seed Martin Klizan.