Rotterdam, The Netherlands (SportsNetwork.com) - Fifth seed Grigor Dimitrov was a hard-fought opening-round winner Monday at the $1.825 million ABN AMRO World Tennis Tournament.

The Bulgarian star overcame French veteran Paul-Henri Mathieu, a qualifier this week, 4-6, 7-6 (7-2), 6-2 at Ahoy Rotterdam.

An upset came when promising Austrian Dominic Thiem dismissed sixth-seeded Latvian Ernests Gulbis 6-4, 6-2.

Also on Day 1, seventh-seeded Spaniard Roberto Bautista Agut drubbed German wild card Alexander Zverev 6-4, 6-1, Italian Simone Bolelli subdued Czech Lukas Rosol 6-3, 6-1 and Gilles Muller topped David Goffin 7-6 (7-5), 6-7 (6-8), 6-3.

This week's top seeds are Australian Open runner-up Andy Murray, Milos Raonic, former Wimbledon finalist Tomas Berdych and last year's Aussie Open winner Stan Wawrinka. The reigning Rotterdam champion Berdych beat reigning U.S. Open titlist Marin Cilic in last year's finale here.