Basel, Switzerland (SportsNetwork.com) - Two of this week's top-four seeds -- Tomas Berdych and Stanislas Wawrinka -- suffered opening-round upsets Tuesday at the $1.955 million Swiss Indoors tennis event.

Towering Croat Ivo Karlovic toppled the second-seeded Berdych 4-6, 7-6 (7-4), 7-6 (7-2), while Frenchman Edouard Roger-Vasselin took out the fourth-seeded Wawrinka 6-4, 6-3 at St. Jakobshalle. Karlovic and Berdych combined for 34 aces, including 23 from the powerful Croat.

Sixth-seeded Japanese Kei Nishikori, the 2011 Basel runner-up, avoided an upset by tackling Swiss wild card Marco Chiudinelli 6-2, 6-4, while another upset came when German Daniel Brands dismissed seventh-seeded Italian Andreas Seppi 7-6 (7-3), 6-3.

In other first-round action, Uzbekistan's Denis Istomin topped Argentine Horacio Zeballos 7-5, 7-6 (7-3), Croat Ivan Dodig was beating Carlos Berlocq 2-0 in the opening set when the Argentine retired, and Marcos Baghdatis grounded German qualifier Benjamin Becker 7-6 (10-8), 6-1. The former Australian Open runner-up Baghdatis was a Basel finalist in 2005.

In some more opening-round action here on Wednesday, top-seeded and defending champion Juan Martin del Potro will take to the court, while a second-round affair will pit third-seeded former world No. 1 Swiss icon Roger Federer against Istomin.

Federer is a five-time Basel champion and four-time runner-up, with the his last title coming in 2011. He lost to his fellow former U.S. Open champ del Potro in last year's marquee finale.

The 2013 Basel winner will earn $475,000.