to closely guard or track an opposing player in a sports context

to follow someone secretly in order to observe their movements

to gaze fixedly or intently at someone or something

to keep a close eye on; to monitor

to keep a close watch on or follow a situation or person's actions closely

to stare fixedly, intently, or relentlessly at someone or something

to keep a close eye on someone (idiom)

mark to market (accounting)

to keep a close watch or maintain strict surveillance over someone or something

to fix one's eyes on someone or something; to watch intently