Cilic returns to winning ways following ban
Croatia's Marin Cilic hits a return on April 18, 2013, in Monaco
Cilic, 25, currently ranked 47th in the world, tested positive for the stimulant nikethamide at the Munich Open in May and was banned for nine months by an independent tribunal in September.
However the Court of Arbitration for Sport (CAS) reduced the ban to four months last Friday meaning the sanction expired one day later and allowed him to take his place in the tournament.
After dropping a tense opening set, Cilic bounced back to set up a matchup against world number five Juan Martin Del Potro of Argentina with a 5-7, 6-1, 6-4 victory.
World number one Rafael Nadal who is not in action on Monday said he was pleased to see Cilic back on tour.
"One thing I can say is I'm happy to see Marin back on tour. He's a good guy and a great player.
"I don't know what happened but if he's back, it's because it's fair that he's back. That's all. I'm happy for that."
Also on Monday in a programme which featured only unseeded players, Spaniard Feliciano Lopez edged a fiercely contested match against Australian qualifier Bernard Tomic 6-4, 6-7 (4/7), 7-6 (7/1).
Lopez will now meet Stanislas Wawrinka with the Swiss seventh seed looking to wrap up his first ever appearance at the London Masters with a good run this week.
Wawrinka is battling with compatriot Roger Federer, and French duo Richard Gasquet and Jo-Wilfrid Tsonga for the final tickets to the eight-man tournament in the English capital.
Elsewhere, Jeremy Chardy became the first French player to fall by the wayside as he was well beaten 6-3, 6-4 by Czech Lukas Rosol, while his compatriot Adrian Mannarino also crashed out in a tough three set defeat against Colombian Santiago Giraldo 6-3, 2-6, 6-4.
Dutchman Robin Haase clinched the first set 10-8 in a tie-break before closing out his first round match 7-6, 6-3 over Denis Istomin of Uzbekistan.