Commentators and tennis fans alike were left confused by a metal disc strapped to Novak Djokovic's chest during his French Open victory over Marton Fucsovics.
The World No 3 beat his Hungarian opponent in straight sets 7-6(2), 6-0, 6-3 to progress to the third round to face Spaniard Alejandro Davidovich Fokina.
While Djokovic's quality eventually told, he didn't have it all his own way in the opening set.
He took an early-break lead but the reigning French Open champion was broken back when serving for the set.
After an hour and 27 minutes, Djokovic finally sealed the first set in a tiebreak.
Unsurprisingly, after being made to sweat for the set, Djokovic required a change of shirts.
In doing so, however, he showed off a rather strange metal disc strapped to his chest.
It looked like a little battery and had the commentators baffled.
"Something taped to Novak," former pro Jim Courier said while commentating on the match for Tennis Channel. "Have you ever seen Robert Downey Jr. in the same room as Djokovic? Maybe Novak is Iron Man," he joked.
Later in the match, he was passed another metal disc by a ball girl - presumably given to her by Djokovic's team.
He removed the tape and replaced the metal disc.
"His trainer did give the ball girl another one of these," Jason Goodall said. "Novak a little confused as to whether to go over the top or under the bottom. He did change it in the end."
So, what was the metal disc and why was it strapped to his chest?
Well, Djokovic was asked about it after his victory to which he replied: "When I was a kid I liked Iron Man a lot, so I try to impersonate Iron Man."
He went on to sarcastically add: "My team delivers an incredibly efficient nanotechnology to help me deliver my best on the court, so that's the biggest secret of my career. If it wasn't for that, I probably wouldn't be sitting here."
He clearly didn't want to tell people exactly what the metal disc did.
Plenty of tennis fans have been discussing what it actually did on Reddit - assuming that Djokovic is being facetious.
"Possibly a magnet for pain relief," one fan suggested.
Another fan wrote: "Wonder whether it's some sort of fitness tracker or GPS to track his court movement for post-match analysis."
While a third added: "My guess is some sort of biometric tracker. It could be tracking anything from steps, to heart rate or variability, to blood sugar levels."