The 1D X finally met its match Tuesday night. I filled in for a reporter for the News Journal at a game in Mansfield, in a terribly lit gym. I used a 135L with a 1D X at 1/500s, f/2, and ISO 3200 to 6400 depending on location on the court. The majority of my shots were slightly OOF, and I attribute this to the AF system not being able to lock focus in the dim lighting. Looking at my photos, it didn't apparently lock on anything in the frame. In a few burst shots, the first one was in focus (while athlete still on the floor) but the camera lost focus as the player went up for the layup, even with face-recognition on. Needless to say, as I'm going through my RAW files right now, I'm highly disappointed. I will admit though, that the 135L is not as good at locking on focus as the 70-200 f/2.8L II IS zoom lens, but I couldn't afford the loss of a whole stop of light in this gym. I know I've been spoiled with a lot of light lately, but I just wanted to point out that as good as the 1D X is, and it is, it is certainly far from perfect. My suggestion is that in these cases maybe don't use high burst rates, but wait for a moment and try to lock on, then move the camera with the player, sort of like panning, and then fire a shot, maybe even just one shot. Bursting did NOT work at all here. Ok, I'm done venting.
Your story is interesting to me because I have been shooting primarily (personally not professionally) high school basketball for the last several weeks in a variety of gym lighting conditions using the 1DX, and I have not experienced the problem you ran into. I know from past years that some gyms have truly horrible lighting conditions, worse than what I've been shooting in lately, so I imagine that the symptom you describe may be both setting and lens dependent. I shoot my 1DX with either the Canon 24-70 f/2.8 L or the 70-200 f/2.8 L IS. The latter seems to track focus better and allow faster burst rates, but both have reliably achieved focus quickly. I shoot in AI servo, with the high speed burst rate set to 10 fps (not that it always achieves that). I also have the first and second frame focus priority settings set to focus priority (i.e. "don't fire unless focus is locked"). Sometimes that means the camera takes a fraction of a second before firing, if it hasn't locked on, but it has usually meant that the shots are in focus (unless I screwed up and put the focus point on the crowd). I also set the camera in aperture priority mode, with an aperture of 4 to 5.6 and a minimum shutter speed of 1/500th and the ISO set to Auto. This has resulted in ISO settings of as high as 20000 in some cases, and I usually need to apply 50-60% luminance noise reduction in Lightroom, but the pictures end up looking great. Don't know if any of that helps, but thought I'd offer my recent experience.