to watch sb closely; to breathe down sb's neck; to mark (sports)

to keep an eye on

to follow sb; to tail; to shadow

to gaze; to stare fixedly

mark to market (accounting)

to gaze intently at; to scrutinize

to stare fixedly; to look concentratedly

(sports) to closely guard an opponent; (fig.) to keep a close eye on sb; to monitor closely