Mathieu lost just one game in taking the first set from Marin Cilic but faltered from there, losing 1-6, 6-4, 6-3 to the No. 55-ranked Cilic.
Defending champion and fourth-seeded Ivo Karlovic defeated Jonas Bjorkman 6-3, 6-4 while No. 6-seeded Dmitry Tursunov took a 7-5, 6-4 win from Yen-Hsun Lu and seventh-seeded Andreas Seppi came back for a 3-6, 6-4, 7-5 win over Dudi Sela.
Querrey advanced to a second-round match against Karlovic.
Top-seeded Radek Stepanek opens his tournament play Tuesday against John Isner.
Notable deaths of 2014 [PHOTOS]