John McEnroe believes 23-time grand slam champion Serena Williams choked in her Australian Open quarter-final defeat to Karolina Pliskova.

Williams surrendered a 5-1 lead in the final set and wasted four match points as she sensationally bowed out 6-4 4-6 7-5 against Pliskova at Melbourne Park on Wednesday.

Former world number one Williams was bidding to equal Margaret Court's slam record of 24 titles but the 37-year-old failed to see off Pliskova.

McEnroe – a seven-time major champion – was forthright in his assessment of the American star's elimination.

"As you get older, you realise you have less opportunity to maybe pull this off. So you put more pressure on yourself," McEnroe told Channel Nine.

"It looked like at the end, the pressure, which is – very rare, I didn't think I would be saying this – but the pressure of the moment got to her a bit.

"[It's] a little bit of choking, yeah. Let's face it, they're all human beings."

McEnroe was also scathing of Kei Nishikori's retirement against Novak Djokovic in the men's quarter-final at the Australian Open.

Nishikori was trailing 6-1 and 4-1 before he succumbed to a right quad problem following his marathon exertions at the tournament.

McEnroe, however, was not sympathetic towards the Japanese eighth seed as he questioned his mental strength.

"You should read into it that it's sort of like a middleweight playing heavyweights. At a certain point, he gives in mentally," McEnroe said.

"That's why he hired [Michael] Chang. Michael wouldn't do that, and you could see Michael was dismayed in the coach's box; 'Why do you have to sort of stop playing?'

"That's not in his [Chang's] DNA, so he's trying to get that more into Kei's. But Kei ... some guys and girls have higher thresholds of pain than others and they can go out there and compete at a higher level for longer. But he's not one of them."