ROTTERDAM, Netherlands (AFP) — Australian Open champion Jannik Sinner overcame a stubborn fightback by veteran Frenchman Gael Monfils to battle into the Rotterdam Open quarterfinals with a 6-3, 3-6, 6-3 win while other top seeds crashed out on a day of upsets.

The 22-year-old Italian, unbeaten this year, showed the imperious form that took him to victory in Melbourne from the start, racing into a 3-0 first-set lead.

Wildcard Monfils, 37, entertained the crowd with some trademark retrieving and the occasional trick shot but initially had no answer to Sinner's consistency, losing the first set 6-3.

But the Frenchman, now 70th in the world, rolled back the years in the second set, delighting the Rotterdam crowd that saw him win back-to-back titles in 2019 and 2020.

Cleverly mixing blistering forehands and well-judged drop shots, he disturbed Sinner's rhythm and broke serve early in the second set, hanging on to close it out 6-3.

A poor Monfils first service game in the deciding set, riddled with unforced errors and a double-fault, handed the initiative back to the Italian, who again opened up a 3-0 advantage.

Sinner's relentless pressure told and the world number four eventually broke the French resistance, finishing off the third set 6-3.

"It's always tough to play against him. He won two times here, so he knows how to play here," Sinner said.

"I'm very happy. I was struggling a bit, I didn't have so much rhythm."

"But in these kinds of matches you learn a lot. It gives me confidence for the next one," against Canada's Milos Raonic.

Earlier in the day, rising star Holger Rune suffered a shock defeat to Kazakh Alexander Shevchenko and local hero Tallon Griekspoor beat fourth-seed Hubert Hurkacz in a thriller with the help of raucous crowd support.

Both winners said the victories were their "biggest" or "best."