Pella, ranked No. 101 in the world, eliminated top-seeded Janko Tipsarevic, ranked 10th, 7-6 (7-1), 6-1. Pella took two-thirds of the points on serve and faced one break point. After the lopsided tiebreaker, he lost only five points while serving in the second set and took all but seven when Tipsarevic served.
Igor Sijsling also advanced with an upset, ousting fourth-seeded Philipp Kohlschreiber 3-6, 6-3, 6-4. Sijsling had 13 aces and saved the only two break points he saw after the first set.
Other seeded players headed to the quarterfinals are No. 6 Jarkko Nieminen, who beat Roberto Bautista-Agut 6-4, 5-7, 6-3, and seventh-seeded Viktor Troicki, who defeated Grega Zemlja 6-3, 7-6 (7-3).
Also Wednesday, Jan Hajek routed Daniel Brands 6-0, 6-1 and Tobias Kamke advanced over Aljaz Bedene 6-1, 6-4.
Thursday's quarterfinals have Pella going against Troicki and Monaco playing Kamke in the top of the draw. Hajek meets Sijsling and Nieminen faces Haas in the other matches.