An incident involving Alex Michelsen at the Auckland Classic has sparked intense debate over the application of tennis rules.

The American, 20, hit a shot long from the net in his round-of-16 clash with Lorenzo Sonego to go four games down in the opening set. He responded by taking a ball out of his pocket and pelting it into the stands, where it hit a spectator.

Michelsen, who will compete at the Australian Open later this month, immediately looked mortified and frantically offered his apologies. Despite similar incidents seeing players disqualified in the past, the fan confirmed that they were unharmed and Michelsen was permitted to see out the match.

The fairness of that decision has been called into question by former tennis star Prakash Amritraj, who highlighted a famous incident involving Tim Henman which saw him harshly defaulted on the spot.

"It's brutal to have something of such high stakes rely on that, but at the same time, firing a ball in anger is always taking a risk," Amritraj told the Tennis Channel.

"Tim Henman, the nicest individual you will ever meet, fired a ball into the net, literally into the net in the 90s at Wimbledon. A ball kid was running across and it happened to hit her. Defaulted on the spot. Call it bad luck, it's tough either way but it's running a risk.

"Alex Michelsen is a really good kid, keeps his head down, works really hard, nice to all the other players. If he gets defaulted here, that's going to stay with him for months."

Michelsen went on to defeat Sonego in straight sets, setting up a clash with fellow American Nishesh Basavareddy in the quarter-finals.

His brush with disqualification came after Cameron Norrie found himself in a similarly dicey situation at the same tournament. The British tennis star launched his racket in frustration after going down match point in his defeat to Facundo Diaz, and it hit a fan in the stands behind him.

Norrie was given a warning by the chair umpire before being allowed to continue. Afterwards, he admitted it was completely out of character, saying: "I wasn't meaning to do that but it's still not ideal to be doing that and I've never done something like that.

"As we saw with some other players, you can easily be defaulted if it catches them in the wrong spot or they're not looking or something. I was not meaning to do that and it is completely not me to do something like that. I apologised very quickly and I want to apologise in general. I'm not happy with how I behaved."