With so much on the line - the golden slam, the calendar-year grand slam, Olympic singles gold - it's a mystery to many why Novak Djokovic decided to enter the mixed doubles tournament at the Tokyo Olympics.
Why risk injury or getting fatigued because of an event that doesn't have much of a following and might distract from the more pressing goals within reach?
"I was against it. The whole team was against it," Viktor Troicki, Serbia's Olympic team coach and Davis Cup captain, revealed after Djokovic reached the semifinals in both singles and mixed on Thursday.
"It was just him. He wanted to play."
So there Djokovic was, partnering with Nina Stojanovic to beat the German pair of Laura Siegemund and Kevin Krawietz 6-1 6-2 in a match that started less than two hours after he routed Japan's Kei Nishikori 6-2 6-0 in the singles.
"I thought he had enough of tennis the last months," Troicki said. "Really, he played so much.
"Everyone was thinking about the singles. When he said he wanted to play mixed, I was like, 'OK, (if) it means that much to him and he wants to prove that he's ready and in the mixed matches he's giving everything, he's 100% focused on going all the way, then OK.'
"He's looking for a harder challenge," Troicki added with a laugh.
"It's working well so far. I'm glad he's having short matches in both events."
Djokovic hasn't lost a set yet in singles or mixed doubles.
Yet Djokovic will face his biggest test of the Olympic tournament against fifth-ranked Alexander Zverev on Friday - even though Djokovic holds a 6-2 career edge over the German and has not lost to Zverev in nearly three years.
"It's probably the final before the finals," Troicki said. "It's going to be an interesting match."
Karen Khachanov and Pablo Carreno Busta will play in the other semi-final.
The 34-year-old Djokovic is already the first man since Road Laver in 1969 to win the first three major tournaments in a season.
"Already now, it's obvious that he's the greatest in our sport," Troicki said. "The greatest ever to play and to achieve so many records.
"They're both great champions, both Rafa (Nadal) and Roger (Federer), and I admire what they've done.
"But really Novak has (reached) new heights and new records and he's for sure not going to stop here. He's going to prove that he's the best ever player.
"He's mentally ready for everything. What he's doing in our sport is something special and he's mentally tougher than anyone ever in this game."
Australian Associated Press