Dutch qualifier Igor Sijsling stunned third seed Jo-Wilfried Tsonga 7-6 4-6 6-4 in the first round of the ABN AMRO World Tennis Tournament in Rotterdam.
After winning an opening set tie-break, world No.77 Sijsling regathered his composure after losing the second set to record one of the biggest wins of his career against the former Wimbledon finalist.
Meanwhile, Tsonga's French compatriot Gilles Simon started his campaign with a 7-6 6-4 victory over Belgian qualifier Daniel Brands.
After an indifferent start to the match in which both players lost their serve twice, the more experienced Simon showed his quality to win the tie-break 7-3.
The fifth seed needed only one break of serve in the second to take the match in 92 minutes.
Earlier in the day, seventh seed Jerzy Janowicz was knocked out by Romanian Victor Hanescu, who won 7-6(4) 6-3.
In other results on Monday's second round, Australian youngster Bernard Tomic lost 6-3 3-6 6-3 to Grigor Dimitrov, but Matteo Viola beat Marcel Granollers 5-7 6-3 6-1 and Martin Klizan beat Paul-Henri Mathieu (France) 6-4 3-6 7-5.
World No.2 Roger Federer begins the defence of his title on Wednesday.