It was an afternoon of serious upsets in the quarter-finals of the Australian Open Junior Championships, with three of the top four boys tumbling out of the tournament.

Yi Zhou, the eleventh seed from Beijing, caused the shock of the day when he dethroned top seed Kilian Feldbausch in a two-hour-29-minute lunchtime cracker of a match.

Zhou, who missed a match point on serve at 5-4 in the second set, was forced to play on for almost another hour before finally coming home 7-5 5-7 6-4.

Swiss star Feldbausch received on-court physio treatment midway through the second set but Zhou proved mentally resilient to hang in and now plays third-seed Alexander Blockx in Friday’s semi-final.

Blockx, from Belgium, was the hardest worker of the day by some distance.

His first match of the day was his singles quarter-final on show court 3 against Brazilian Joao Fonseca, who is also his doubles partner this week. 

His 6-7 7-6 6-3 win could have easily gone the other way, he said.

“He is an amazing player," acknowledged the Belgian. "I could have lost 7-6 7-6. He missed a really important volley at 5-5 to get me confidence. I’ve known Fonseca for a year, he has progressed really well, he didn’t play a lot before last year.”

It being Australia Day - a public holiday - the morning was lit up as six Australian Air Force aeroplanes with coloured air trailing in their wake ‘dive-bombed’ Melbourne Park for about 10 minutes as part of the Australia Day acrobatics display. 

The noise above Court 3 was considerable but the Belgian blocked it out easily, with the added benefit that his opponent was unable to do the same.

“It affected him, I broke him and the game after that lasted two minutes,” said Blockx who has recovered well from an injury sustained in last week’s junior Australian Open warm-up event.

“I am really happy to be playing the semi-finals. I hurt my right arm in Traralgon last week,” he said.

He will perhaps be happier facing the in-form Zhou, who he has never previously faced, than the now deposed top seed Feldbausch.

“I’ve played Kilian twice,” he smiled. “Lost twice.”

Before Friday’s semi-final came a doubles match for Blockx with Fonseca, again on court 3 and which was won on a long tie-break which meant that the Belgian then played a third match of the day (the doubles semi-final against the Australian wildcards pair Errey and Schoeman) again on court 3.

Back to the singles and there was a further early upset when second seed Iliyan Radulov was blown away in just 40 minutes by the American Learner Tien, 6-0 6-0.

Tien, a lefty from California and who played in the first round of last year’s US Open, now faces the unseeded 16-year-old Tomasz Berkieta in the semi-finals. Berkieta, is ranked 36 and put out fourth seed Arthur Gea in exactly two hours, winning 10-8 on a final set tie-break.

On match point down Gea put an overhead long and wide and promptly smashed his racquet into the ground. Handshake done, an overjoyed Berkieta looked skywards with his right index finger pointing up and swirled around in a congratulatory dance, absolutely thrilled.

Tien though will be the strong favourite to overcome Berkieta in the last four on Friday.