Tomas Berdych managed to win an AMS having just turned 20, in 2005. Regardless of what any MTF recency bias experts say (and there are many, or many who simply don't remember), he played arguably the tournament of his life and as BodyServe said, he was beating Roger Federer aged 19. He was beating Rafael Nadal in AMS events (defending champion in Madrid and Canada 2006), who was no worse at that stage than many other stages in his career when he was losing to the likes of Jurgen Melzer, Florian Mayer, Ivan Dodig and co.
The argument is that it didn't need to be his "pre-prime". Tomas Berdych beat Roger Federer and Novak Djokovic at Wimbledon in 2010 - he didn't just all of a sudden become that player in that event, he was that player the years prior. That's why I asked where he was.
Yes, he was an incredibly consistent performer overall for a non-slam winner, which as I said, made it all the more incredible that it was becoming increasingly obvious that he was never going to win another big title. Normally, as players become more consistent, they create more opportunities and at some point in time one of those opportunities will materialise in a big tournament win. The opposite seemed to happen with him. At least, that was my perception and it turned out to be right on that one.