The biggest surprise came as Jiri Vesely knocked out eighth seeded Dutchman Igor Sijsling, a player ranked 52 places above the Czech, 7-6 (7-2) 6-3.
Vesely dominated the second set after edging the first on a tie-break, and expressed satisfaction at taking his place in the second round of the ATP 250 clay-court competition.
"I played Igor in Indian Wells (in March) and it wasn't an easy match," Vesely said.
"Today on clay he was playing even better. He was making me run. I was very happy that I was serving really well and that I was able to win."
Home hope Dustin Brown was another surprise victor, serving eight aces and breaking fifth seed Lu Yen-Hsun of Chinese Taipei four times on his way to a 7-6 (7-3) 6-2 success.
Serbian wildcard Nikola Milojevic emerged triumphant in longest match of the day, overcoming Mirza Basic of Bosnia-Herzegovina 6-2 5-7 6-4 in two hours and three minutes.
But there were no such complications for Austria's Jurgen Melzer, who needed just an hour to dispatch Pablo Carreno Busta of Spain 6-4 6-1.
Elsewhere in the draw, Russian Teymuraz Gabashvili beat Serbia's Dusan Lajovic 3-6 6-3 6-4.