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"We are tennis players and we try to win every match we play but it is a special one," Bublik said. "I had never beaten a No. 1 in the world. It is an accomplishment."

Bublik lost to the Italian in the Roland Garros quarter-finals earlier this month but gained revenge in style at the ATP 500 grass-court event. In a red-hot display, the Kazakhstani fired 36 winners, including 15 aces, to overpower the top seed and earn his fourth Top 10 win after two hours and four minutes.

"I kept serving. I tried to be clutch. I tried to get back whatever I [could]. He is an unbelievable player and I was not thinking that I could beat him," said Bublik, the first player since last August to defeat Sinner other than Carlos Alcaraz. "But I had a few chances and executed them well. I kept serving. I had better chances than at Roland Garros. I stayed there, tried to return as much and serve aces, so the ball doesn't come back and it worked really well."

Sinner had won his past 66 matches against players outside the Top 20 in the ATP Rankings, dating back to August 2023. However, he was unable to cope with Bublik’s aggression in the pair’s sixth meeting (4-2 Sinner). It is the first time Sinner has failed to reach a final since Montreal in August last year.

The Italian, who won his maiden grass-court tour-level title in Halle last season, arrived in Germany off the back of his five-hour, 29-minute five-set Roland Garros final defeat to Alcaraz. Sinner will next head to Wimbledon, where he reached the semi-finals in 2023.

Sinner defeated Yannick Hanfmann in straight sets in his opening match in Halle and looked in control during the first set against Bublik. He did not face a break point to move ahead after 41 minutes. However, momentum started to swing at the start of the second set when Bublik crucially saved three break points in his opening service game.

After holding, the World No. 45 gained the decisive break in the sixth game when Sinner framed a forehand long off a cleanly-time backhand return from the World No. 45.

Bublik struck 20 winners in the set to shock Sinner and level and he continued to open his shoulders in the decider. The 27 year-old broke Sinner’s serve once again when he nailed a scorching forehand pass and he closed out the match at the first time of asking with a smash.

"I had an amazing forehand to break in the third set and then I kept serving," Bublik said. "On grass, if you lose your serve, it is maybe tough and I am a guy who doesn't lose a lot of serves. I just kept serving and putting him in uncomfortable positions and it worked well."

Bublik has fond memories in Halle, having captured the biggest title of his career at the tournament in 2023. Chasing his fifth tour-level crown, the 27 year-old will meet Tomas Machac in the quarter-finals after the seventh-seeded Czech beat Fabian Marozsan 6-2, 6-3.

Second seed Alexander Zverev survived a stern test from Lorenzo Sonego to become the sixth player in tournament history to record 20 wins at the tournament.

The German made 90 per cent of his first serves in the match but struggled to make a dent on Sonego’s serve in a two-hour, 25-minute epic.

With little to separate the pair, Zverev prevailed 3-6, 6-4, 7-6(2) to reach the quarter-finals in Halle for the sixth time. The No. 3 in the PIF ATP Rankings is chasing his first tour-level title and grass and will next meet Flavio Cobolli.

Cobolli came through his second deciding-set tie-break of the week to move into the quarter-finals. After saving a match point to defeat Joao Fonseca in the first round, the Italian held firm for a 7-6(2), 4-6, 7-6(5) victory over Denis Shapovalov.

Tomas Martin Etcheverry saved two match points before upsetting fourth seed Andrey Rublev 6-3, 6-7(4), 7-6(6) to reach his first tour-level quarter-final on grass. 

Rublev saved a match point on serve at 4-5 in the second set and then held two of his own at 6/4 in the third-set tie-break, but Etcheverry reeled off the final four points to seal victory.

Etcheverry will next face eighth seed Karen Khachanov, who ousted Felix Auger-Aliassime 6-3, 1-6, 6-3.

The 27 year-old Rublev, who reached the Halle final in 2021 and 2023, is 15th in the ATP Live Race To Turin. He is aiming to qualify for the ATP Finals for the sixth consecutive year.