Nick Kyrgios faces a potential four-week ban from the ATP Tour after landing in hot water yet again during an explosive second-round exit from the Shanghai Masters.

Effectively on six months’ probation, Kyrgios was issued with an unsportsmanlike conduct warning during his 1-6 6-4 6-4 loss to Japanese sixth seed Kei Nishikori.

Chair umpire Mohamed Lahyani sanctioned Kyrgios after the Australian hot-head fired a scorching practice shot down the middle of the court after serving a fault in the opening point of the deciding set.

A linesman had to jump out of the way to avoid being struck, prompting Lahyani to issue Kyrgios a warning that potentially carries a maximum $US10,000 ($A13,771) fine.

Kyrgios is serving a six-month suspended sentence for his unsavoury sledging of French Open champion Stan Wawrinka at the Montreal Masters in August.

If he racks up fines totalling $US5000 or more before February, the 20-year-old will be banned from the ATP Tour for 28 days.

The two-time grand slam quarter-finalist was slapped with a $1500 fine on Tuesday for a foul-mouthed outburst during his first-round win over Andreas Haider-Maurer.

Kyrgios’s latest antics overshadowed an impressive 6-4 6-2 second-round win earlier on Tuesday for fellow Australian Bernard Tomic over Spanish seventh seed David Ferrer.