In order to hit "forcing shots" and "winners constantly" and win; a player has to get to the ball, get in position to hit the right shot, hit the shot, recover and get in position to hit the next shot. And hit most shots with power and usually spin. Serving and volleying, taking trips into the net to close points, backing up to hit overheads, etc. This all takes a lot of energy. Especially if the opponent is a consistent defensive player. And then it's 90 degrees and 70% humidity at match time.
Being a 4.5 and higher isn't about hitting winner after winner. It's real tennis just like you see on TV...if you're playing legit players. Of course I live in one of the biggest tennis towns in the USA and there are never any gimme matches in singles. Nobody gives up and everybody retrieves everything, especially on clay. I can't remember the last time I played someone who didn't fight tooth and nail to the finish. I'm a solid player with a complete game who can dominate anyone who isn't a high 4.5. Until I get tired.
Then I start getting to the ball a little late or am slightly out of position. Errors creep in. Gets harder to push with legs into serve, etc. The other player starts to scramble even harder hoping to capitalize.
Someone would have to be out of touch with real competitive tennis to think that fitness isn't the cornerstone of consistent winning tennis above the 3.5 level. Or play in an area with a bunch of hacks who cant hit more than two real shots in the court during a point.