Roger Federer's path to the world No.1 ranking just took a turn down an easier route after three-time grand slam winner Stan Wawrinka blew a lead and lost to an unfancied Dutchman in Rotterdam on Tuesday.
The fifth seed Wawrinka took the first set but was beaten by Tallon Griekspoor, who won the first-round clash 4-6 6-3 6-3.
The 32-year-old slumped to another surprise defeat by an outsider, after losing in the Australian Open second round to Tennys Sandgren and in last week's Sofia Open semi-finals to Bosnian Mirza Basic.
Wawrinka, the 2015 Rotterdam winner, is struggling to regain top form after a pair of summer knee operations.
The Swiss has now lost three matches and won three since returning in January, his devastating single-handed backhanded of little use as he struggled with his movement.
His compatriot Federer, 36, needs to reach the semi-finals to become the oldest male in history to reach the No.1 ranking, and an important quarter-final with his great friend Wawrinka loomed.
But the 21-year-old Griekspoor, ranked 259th in the world, earned the biggest success of his career as he competed in only his third ATP-level match.
Federer begins his campaign against qualifier Ruben Bemelmans, with world No.42 Robin Haase now his highest-ranked potential quarter-final opponent.
Meanwhile, Belgian fourth seed David Goffin, last year's runner-up, brushed aside Nicolas Mahut in straight sets to reach the last 16.
Goffin came through 6-1 6-3 to post his fourth victory over the Frenchman in five meetings.
The world No.7 Goffin totally dominated as he set up a second-round tie with Spaniard Feliciano Lopez, winning in just 64 minutes.
Earlier on Tuesday, ninth seed Gilles Muller of Luxembourg lost to Russian qualifier Daniil Medvedev 6-4 7-6 (11-9).
Serb Viktor Troicki saw off Jan-Lennard Struff 1-6 7-6 (7-5) 6-2, while Struff's fellow German Philipp Kohlschreiber fought back to beat Russia's Karen Khachanov 3-6 7-6 (7-1) 7-6 (7-5).