Against the rules, "tanking" is when a player does not try hard enough in order to save energy, or otherwise gain advantage over other competitors. The Chinese badminton team were disgraced by blatant "tanking" during the Olympics and that would be the most obvious example.
Roger Federer avoided playing in Paris to save energy for the World Tour Finals' at the O2 which is in itself a form of tanking, but when he announced that he would treat his last encounter in the Round Robin of the current O2 against DelPotro as a "practice" match, because he had already qualified, that would be a foolish admission of his failure to abide by the rules. Or, put it another way, cheating.
In-form Paris winner Ferrer was not pleased because he needed Federer to win against DelPotro in order to have a chance of qualifying for the semi-finals.
Federer subsequently lost his match, and Ferrer had an out of character hissy fit during his match against Tipsarevic This match was rendered irrelevant and Ferrer attacked the O2 court with his racquet.
Federers admitted "tanking" had hurt Ferrer, and also the sell out crowd at the O2 who were ripped off, someone said.
There may be a failure of the format that encourages bad sportsmanship, but it does not excuse it.