Shonen Jump's attempt to bottle lightning.
ComiPress have a translation of an article on Shonen Jump's appeal to the Fujoshi market which pinpoints Prince of Tennis as a turning point in their marketing.
Prince of Tennis success in appealing to a female audience with over active dirty minds while not alienating the core make audience, can be seen in the look of other recent Jump manga such as Death Note.
In fact looking at Jump's website there are few recent series that err towards the more idiosyncratic designs of a Dragonball or One Piece. It's pretty much just Eyeshield 21 and the assorted gag manga like Bobobo and Jaguar that plough that furough. Even long runners like One Piece are not immune to the effect of looking towards this market. Just look at the recent startling redesign of two certain characters in the manga.
The manga of Prince of Tennis began in 1999, with the anime following in 2001 and running for 4 years. There's also a live action film and a series of Musicals. The musicals appear to involve singers miming playing tennis matches while singing.
From what little I've seen of the series it goes down the route of genius destroys all competitors due some unearthly gift he has to learn to appreciate. Which is my least favourite of the two sports manga plots (the other being the plucky underdog struggles to success, or not. One nice thing about manga plucky underdog stories is that unlike Hollywood stories of the same type, they don't always win).