When Q3 came out everyone was bored of single player. It looked like the whole industry was going that way and that single player FPS was pretty much dead. Then Half-Life was released and blew everyone's minds. I don't think anyone has been able to craft an experience like HL to this day, it's like they just "get" how interactive fiction should feel rather than being a shoot-em-up interspersed with cutscenes.
Not exactly, Unreal the single player game is also from 1998.
If you meant Unreal Tournament, it was released November 22nd 1999, just a few days before Quake 3 Arena that was released on December 2nd 1999. UT was also mostly a multiplayer arena type game like q3a. It was a really hyped up battle of giants back then between those two. Good times.
EDIT: corrected release dates after checking wikipedia
