World number one Jannik Sinner's bid to win back-to-back Halle Open titles was ended on Thursday following a 3-6 6-3 6-4 defeat by Alexander Bublik in the last 16, the top seed's first defeat to a player outside the Top 20 since 2023.

Going into the contest Sinner held a 4-1 win-loss record against Kazakhstan's Bublik, including a recent victory at the French Open. However, in Halle, Bublik now holds a 2-0 advantage over his Italian rival having also beaten Sinner en route to winning the grasscourt event in 2023.

The Italian began brightly as he sailed into a 3-0 lead on his way to winning the first set but eventually came unstuck as Bublik hit 36 winners, including 15 aces.

"I don't know. I kept serving. I tried to get back whatever I can," world number 45 Bublik said when asked how he turned things around.

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"He's an unbelievable player. I was not thinking I could beat him.

"If I would have a few chances and I executed them well and I kept serving, I had for certain a better chance than at Roland Garros," he added referring to his quarter-final defeat at the claycourt major.

Bublik saved three break points in the first game of the second set before gaining the crucial break to lead 4-2 and smashed three aces when holding serve to force the decider where a break to go 4-3 up proved decisive.

"It's a very fast surface. He gave me a chance. I had an amazing forehand to break him in the third set," Bublik said.

"I just kept serving and putting him in uncomfortable positions. It worked well."

Sinner's last loss to a player ranked outside the Top 20 came against Dusan Lajovic, then number 66 in the world, at Cincinnati in August 2023.

Bublik will meet Tomas Machac in the quarter-finals after the Czech seventh seed eased to a 6-2 6-3 win over Hungary's Fabian Marozsan.

German second seed Alexander Zverev, still looking to win his first grasscourt title, overcame Italian Lorenzo Sonego 3-6 6-4 7-6(2).

Zverev, a twice beaten finalist in Halle, recorded his fifth win over the Italian in as many meetings, having also beaten Sonego at the same stage of last year's tournament. He next faces another Italian, Flavio Cobolli.

Cobolli squeezed past Canadian Denis Shapovalov 7-6(2) 4-6 7-6(5).

Another two-time losing finalist, Andrey Rublev, exits the tournament after a three-hour battle in which the Russian fourth seed was beaten 6-3 6-7(4) 7-6(6) by Argentine Tomas Etcheverry.

Etcheverry, who saved two match points in the deciding tiebreak, will meet another Russian, Karen Khachanov. The eighth seed beat Canadian Felix Auger-Aliassime 6-3 1-6 6-3.