Novak Djokovic may top the world rankings but he insists Rafael Nadal remains the clear favourite to win any tournament on clay.
Djokovic is the top seed for the Monte Carlo Masters but, after winning the Australian Open in January, he suffered surprise early exits in Indian Wells and Miami, losing in the third and fourth rounds respectively.
The 31-year-old from Serbia will hope a switch in surfaces brings about a change in results – he has twice before been crowned champion in Monte Carlo, too.
Nadal, however, is aiming to become the first player to win 12 titles at a single ATP Tour event, the Spaniard fit to compete again after pulling out at the semi-final stage in Indian Wells with a knee injury.
"I think Rafa is obviously always a very clear favourite on any clay court in the world and that doesn't change. He's still there," Djokovic told the ATP Tour.
"If he's physically fit he's definitely the number one guy on this surface, without a doubt.
"After him, it's quite open. Zverev has done really well on clay the past few years. Dominic Thiem, of course. Roger is going to play as well this season, so that's great for tennis. I think it is pretty open."
Djokovic will begin his quest for glory against either Philipp Kohlschreiber or Taro Daniel, while Nadal will face the winner of the match between fellow Spaniard Roberto Bautista Agut and Australian John Millman.
STAN DELIVERS AFTER SLOW START
Former champion Stan Wawrinka was victorious in Sunday's first-round action, battling back from 5-3 down in the first set to beat Lucas Pouille 7-5 6-3.
The Swiss – who lifted the trophy in 2014 – sealed his progression at the third attempt, with Marco Cecchinato or Damir Dzumhur up next.
"The beginning was a bit tough, I was hesitating, I was a bit slow, I was nervous and I was looking a little bit for the conditions," Wawrinka told the ATP Tour website.
"He took advantage, he broke me directly and then I started to play a little bit better. I started to know a bit more what I had to do."
Also in the main draw, Borna Coric overcame Hubert Hurkacz 6-4 5-7 7-5 in a rain-hit contest, while Grigor Dimitrov – a semi-finalist a year ago – and Guido Pella recorded respective victories over Matteo Berrettini and Laslo Djere.