Murray has to dig deep to serve up victory after injury
Andy Murray admitted he found the going tough after marking his return from injury with victory over Gilles Muller in the second round of the Monte Carlo Masters.
The world number one was playing his first match since March 12 after recovering from an elbow problem that affected his serve.
Ahead of the match, Murray insisted he was serving normally again, but three double faults in the first four points of the match got him into early trouble.
He then had to save a set point before reeling off four games in a row and, despite continuing to play some way below his best, battled through the second set to win 7-5 7-5.
Murray said: "It was a tough first match, I started the match pretty slow, I obviously wasn't serving that well at the start.
"I haven't been in the best rhythm on serve because I only started serving at full speed four or five days ago. I knew it would take a bit of time but I wasn't expecting to start serving like that."
Murray will next face 15th seed Albert Ramos-Vinolas, who eased to a 6-2 6-2 win over Carlos Berlocq.
Nine-time champion Rafael Nadal was given a real scare by Britain's Kyle Edmund before coming through 6-0 5-7 6-3.
Nadal had never lost more than four games in a set in his opening match at the tournament but, after breezing through the opener, was blown off course by the power of Edmund's forehand.
The 22-year-old had a chance to lead 3-1 in the decider but Nadal held firm and ultimately proved just the stronger.
Third seed and former champion Stan Wawrinka dropped a set to Jiri Vesely, who defeated Novak Djokovic here last season, but recovered to win 6-2 4-6 6-2.
Dominic Thiem defeated Robin Haase 6-3 6-2, Marin Cilic thumped Jeremy Chardy 6-3 6-0 and Alexander Zverev, who next plays Nadal, saw off Feliciano Lopez 6-0 6-4.
Tomas Berdych defeated 39-year-old Tommy Haas 3-6 6-1 6-4 while Lucas Pouille and Pablo Carreno Busta also won.
World number 11 Grigor Dimitrov, the eighth seed, suffered an unexpected 4-6 6-3 6-2 defeat at the hands of German qualifier Jan-Lennard Struff.
There was also an early exit for 12th seeded Spaniard Roberto Bautista Agut as Argentina's Diego Schwartzman progressed with a 6-3 7-6 (7/3) victory.