There was a troubling sign when TLC hit the stage Sunday at the Baltimore Arena for the sixth stop on the trio’s planned 31-date ”FanMail” tour: Some 3,000 of the Arena’s 12,500 seats were empty for the 90-minute show, adding fuel to industry speculation that the tour’s future is as shaky as the group’s.

TLC mostly hid their internal tensions, performing such hits as ”Waterfalls,” ”Creep,” and ”No Scrubs.” But there were occasional signs of the strife that has divided Lisa ”Left Eye” Lopes from her band mates Rozonda ”Chilli” Thomas and Tionne ”T-Boz” Watkins. At various points during the show, Chilli and T-Boz high-fived each other while ignoring Left Eye. Left Eye, in turn, strutted past the other two as if they were merely set pieces to avoid. Things heated up during a particularly aggressive rendition of ”What About Your Friends” when T-Boz appeared to glare at Left Eye while singing the lyrics, ”What about your friends/Will they let you down/Will they be around?”

Still, TLC proved that the show can indeed go on — and be damned entertaining, too. Most fans seemed oblivious to any problems that may exist between the ladies, and cheered enthusiastically for the trio’s every song. Even so, TLC may soon be facing a bigger dilemma than just trying to get along. Ticket sales have been sluggish. For example, barely 50 percent of the seats for Tuesday night’s concert in Greenville, S.C., had been sold just hours before show time, according to the venue’s director of marketing. (In fairness, the tour was hurriedly launched, leaving barely two weeks for preshow promotion.)

And now TLC has lost K-Ci and JoJo as their biggest-drawing opening act, which won’t help fill seats. One hope does exist, though: Christina Aguilera is expected to join TLC beginning Jan. 3. Perhaps she’ll be just the genie the struggling tour needs.