Daniil Medvedev lost in the first round of the Rotterdam Open, with Roberto Bautista Agut and Grigor Dimitrov also losing on Wednesday.
Medvedev – the 2019 US Open finalist – was playing his first match since a five-set defeat to Stan Wawrinka at the Australian Open but was unable to find the same level he produced in that thriller.
Instead, Medvedev converted just one of his six break point opportunities as Pospisil claimed a 6-4 6-3 victory on Wednesday.
Sixth seed Roberto Bautista Agut was also on the receiving end of a surprise result, compatriot Pablo Carreno Busta winning 6-4 2-6 7-6 (7-4) in the second round.
With Medvedev and Bautista Agut out of the draw, Felix Auger-Aliassime can afford to harbour greater hope of winning his first ATP singles title after he saw off Grigor Dimitrov 6-4 6-2.
Though Auger-Aliassime is a place higher in the rankings than Dimitrov at 21st in the world, some may have expected the Bulgarian’s greater experience to win out.
However, it was the 19-year-old Canadian who controlled matters, enjoying great success against the second serve of Dimitrov, who committed 27 unforced errors as Auger-Aliassime progressed to the quarter-finals.
Felix is firing in Rotterdam!
— ATP Tour (@atptour) February 12, 2020
Daniel Evans will join him in the last eight after the Briton defied the odds to come from a set down and defeat Karen Khachanov 4-6 6-3 6-4.
Gael Monfils, David Goffin and Gilles Simon all sealed first-round wins, though the latter pair did so in unconvincing fashion.
While third seed Monfils eased past Joao Sousa 6-3 6-2, fourth seed Goffin had to come from behind to knock out Robin Haase 3-6 7-6 (7-5) 6-4 and Simon survived a second-set blip in a 7-6 (7-3) 3-6 6-3 win over Mikhail Kukushkin.
Jannik Sinner is also through to round two, the 18-year-old given a walkover after Radu Albot withdrew with a shoulder injury.