Djokovic, playing for the first time under new coach Andre Agassi, cruised past Spain's Marcel Granollers 6-5, 6-4, 6-2 on the clay.

The Serbian recovered from a set down to defeat Andy Murray and complete the Grand Slam of major titles in Paris last year.

But he has since undergone a tumultuous period culminating in a split from his former coaching team.

"He has a lot to share with me outside and inside of the tennis lines, I'm really enjoying it so far," Djokovic said about 1999 French Open winner Agassi afterwards.

He will face João Sousa of Portugal in the second round.

Venezuelan-born Spaniard Muguruza was similarly comfortable despite facing another former French Open winner in 2010 champion Francesca Schiavone of Italy.

Muguruza, who stunned Serena Williams in last year's final, triumphed 6-2, 6-4 to set-up a second round clash with Anett Kontaveit of Estonia.

"I cannot believe in the first round we have two ex-champions," she said afterwards.

"Not only that but Francesca is a legend and I was very excited to play on Philippe Chatrier court with her."

There was more Spanish success as nine-time winner Rafael Nadal began well in his quest for a first major title since his last French Open victory in 2014.

He beat France's Benoit Paire 6-1, 6-4, 6-1 with consummate ease.

Canadian fifth seed Milos Raonic began well with a 6-3, 6-4, 6-2 success over Belgium's Steve Darcis.

Seventh seed Marin Cilic of Croatia also dropped just nine games in a 6-3, 6-3, 6-3 win against Latvia's Ernests Gulbis.

France's Kristina Mladenovic survived two heated arguments with the umpire in a three-hour epic victory over American Jennifer Brady elsewhere in the women's draw.

She was angered by two contentious calls and also had to leave the court in the first set for treatment on a back injury.

The world number 14 and leading home hope trailed 3-0 in the decider before prevailing 6-3, 3-6, 9-7.

Denmark's 11th seed Caroline Wozniacki was another winner after being pushed hard by 17-year-old Australian Jaimee Fourlis.

The world number 337 battled hard but ultimately missed out 6-4, 3-6, 6-2.