Nick Kyrgios should face "severe" disciplinary action after a ball boy was almost struck by the Australian's smashed racket, one of the best-known coaches in tennis has said.

American Brad Gilbert, a former tour player who has worked with stars including Andre Agassi and Andy Murray, was taken aback by the tantrum from Kyrgios after a three-set defeat to Rafael Nadal in Indian Wells.

Kyrgios went to the net to shake hands with Nadal, who won 7-6 (7-0) 5-7 6-4 to reach the semi-finals.

But moments later he thrashed his racket against the ground. It bounced up high, travelling half the length of the court and causing a ball boy to take evasive action, stepping out of the way to avoid being hit and potentially hurt.

Kyrgios scornfully answered reporters' questions about the incident after the match, describing it as "a complete accident".

However, the 26-year-old unmistakably lost control in front of a full stadium and could face punishment as a result.

Gilbert wrote on Twitter: "Ridiculous to do that after such a good match, the penalty should be severe."

Gilbert questioned whether that would happen, given the ATP, which runs the men's tour, only handed a suspended eight-week ban to Alexander Zverev, plus a fine, after the world number three violently hit his racket against an umpire's chair.

"Unfortunately ATP lost the plot with Zverev situation and not a good look for kids and our sport," Gilbert wrote.

The 60-year-old Gilbert, who also coached Andy Roddick, said there was "absolutely no reason to snap on court after the match".

He added: "The stick could have easily gone anywhere like the stands and hit someone, seriously awful to see that in front of great crowd."

Kyrgios posted an exchange of text messages with the ball boy, and said he would give him a racket as a token of apology.

Saying sorry for the incident, Kyrgios wrote: "Didn't want it going anywhere near you to be honest."