He said something I didn't know: that at one point Becker stopped doing this with his tongue; and Andre joked about how panic set in for him then. There's really only one match that he could be referring to, this 1995 loss at Wimbledon, because it was the only time that Becker beat him after 1989 (and so many of us have commented on the panicked look in Agassi's eyes). Their 1995 USO semi was to be their last match until one more Agassi win in 1999, so maybe at the USO Boris was not doing it. Who knows. One thing I do know is how close this Wimbledon match was, regardless.
In "Open," Agassi writes that he wondered during this match if Bollettieri(who was with Becker at the time) revealed to Boris that Agassi knew this 'tell.'