Novak Djokovic holds the Wimbledon title and the number one ranking but this could be the day one slips from his grasp and his grip on the other slackens.
On the other side of the net in the first semi-final will be the man coveting both, as Roger Federer bids to regain his once-secure footing on the top rung of the men's game. If Federer wins his seventh Wimbledon title on Sunday, the number one status will be his again.
"When you're playing at this stage of a grand slam against one of your biggest rivals, there are a lot of things involved, a lot of things on the line," Djokovic said.
"It's not the first time that I'm playing to win or lose a ranking. It's not something that I think about too much. I really just want to focus on the match."
It may seem rather absurd that Federer can leapfrog Djokovic in the rankings, given his last grand slam final was at the French Open 13 months ago and Djokovic and Rafael Nadal have contested all four since then.
But he has been a multiple winner of tour events, if not the majors, and should Federer fight his way past 25-year-old Djokovic on Centre Court, it would surprise many but not the 30-year-old Swiss himself.
The pair will be playing each other on grass for the first time and Federer said: "We don't know quite what to expect. I feel it's a bit of an even ground. I feel good about the match. I'm excited.
"I've been playing well for a year now. I'm happy that going ahead into the semis I'm not tired, I'm not injured. I'm fresh and ready to go. That's how I want to feel before a semi."
Djokovic appreciates Federer is at his most menacing on grass, and had no reluctance to highlight where the threat will be coming from, pointing to his opponent's variety, slice and knowledge of when to strike and when to defend.
"But I have improved playing on grass in the last couple of years," Djokovic said. "I mean, I won the title here last year, got to another semi-final this year."