RIO DE JANEIRO - Even a superstar can be star-struck at the Olympic Village, where Michael Phelps plucked up his courage to get a picture taken with 12-time Grand Slam tennis champion Novak Djokovic.

Phelps, the US swimming juggernaut whose 22 Olympic medals include 18 golds, is usually the man inspiring awe among his fellow athletes. But when he bumped into the Serbian star -- who completed the career Grand Slam at the French Open this year -- it was Phelps who wanted to commemorate the moment.

"I was like, 'Yeah, that's Djokovic," Phelps said. "You give each other the nod and the smile, and I was like 'I'm saying hi. I don't care, I'm saying hi to him. "We ended up talking a little bit about the event schedule and when he was competing and when I was competing and took a photo and wished each other luck and walked away," Phelps said. "He's super-nice, super-down to earth. Easy to talk to. It was something that was cool. I've watched so many of his matches on TV, he's had a lot of epic ones."

Djokovic handed Del Potro revenge clash: World number one Djokovic was handed a blockbuster first round Olympics match on Thursday when he was drawn to face Juan Martin del Potro who beat him for bronze at London in 2012.

Djokovic, whose best performance was a bronze medal at the Beijing Games in 2008, is the top seed at the tournament which gets underway on Saturday.

 The Serb has a 11-3 record against the giant Argetine although the two have not met for three years after Del Potro was sidelined for long periods with a wrist injury.

Second seed and defending champion Andy Murray begins against Serbia's Viktor Troicki while third seeded Rafael Nadal, the 2008 champion, faces Federico Delbonis of Argentina. Japanese fourth seed Kei Nishikori starts against Albert Ramos of Spain.

Defending women's champion and world number one Serena Williams will open against Australia's Daria Gavrilova. Australian Open champion and second seed Angelique Kerber starts against Mariana Duque Marino of Colombia in the first round.