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Tomic, whose world ranking has plummeted to 43, went down to Argentine Diego Schwartzman, who fought back from a 4-2 deficit in the second set to produce a 6-1, 7-6 (7/3) victory.
The Queenslander's last victory was at the Australian Open three months ago, where he reached the third round before losing to Brit Dan Evans.
Tomic struck six double-faults and was broken four times against 41st-ranked Schwartzman, who lost serve only once.
He also lost to Schwartzman in the second round at Istanbul last year.
The Australian is now falling well behind rising compatriot Nick Kyrgios, who is skipping Monte Carlo and starting his Roland Garros run-up campaign in a fortnight at Estoril.
Talented young German Alexander Zverev lost just a handful of games in advancing to the second round by beating Andreas Seppi 6-1, 6-2 in just 68 minutes.
The teenager, who will turn 20 on Thursday, lives in Monte Carlo but only made his debut in the principality tournament a year ago when he won a round.
The 33-year-old Seppi complained he was far from full fitness after dealing with back pain since February.
"My back is in pieces," moaned the 74th-ranked Italian.
In an all-British battle, Kyle Edmund got the better of Evans 7-5, 6-1 to earn a meeting with nine-time tournament winner Rafael Nadal in the second round.
"It's a good opportunity for me, I've never played Rafa," said Edmund. "He's won here nine times, he obviously loves playing here.
"It will be a tough match. I've got to go and give it my best. The flipside -- he's not played me. We have different game styles. But, I've just got to go out there and give it my best."
A pair of Spanish seeds reached the second round, with number 12 Roberto Bautista Agut beating Nicoloz Basilashvili 1-6, 6-3, 7-5 and 13th seed Pablo Carreno Busta outlasting Italy's Fabio Fognini 7-6 (7/0), 6-7 (4/7), 6-3 over almost two and a half hours.
Monte-Carlo Masters results on Monday (x denotes seeded player):
First round
Roberto Bautista Agut (ESP x12) bt Nicoloz Basilashvili (GEO) 1-6, 6-3, 7-5
Alexander Zverev (GER x14) bt Andreas Seppi (ITA) 6-1, 6-2
Pablo Cuevas (URU x16) bt Viktor Troicki (SRB) 6-3, 6-0
Diego Schwartzman (ARG) bt Bernard Tomic (AUS) 6-1, 7-6 (7/3)
Pablo Carreno Busta (ESP x13) bt Fabio Fognini (ITA) 7-6 (7/0), 6-7 (4/6), 6-3
Nicolas Almagro (ESP) bt Martin Klizan (SLO) 4-6, 6-3, 6-1
Kyle Edmund (GBR) bt Dan Evans (GBR) 7-5, 6-1
Adrian Mannarino (FRA) bt Guillermo Garcia-Lopez (ESP) 1-6, 6-3, 6-4
Tomas Berdych (CZE x9) bt Andrey Kuznetsov (RUS) 4-6, 6-3, 6-4
Albert Ramos-Vinolas (ESP x15) bt Renzo Olivo (ARG) 6-2, 6-3
