FORMER champion Milos Raonic lost in straight sets to 18-year-old Alex De Minaur in a second-round upset yesterday at the Brisbane International.

Raonic, who won the Brisbane title in 2016 and reached the Wimbledon final that year, struggled to find rhythm with his usually booming serve in a 6-4 6-4 loss to the Australian wild-card entry.

De Minaur pressured Raonic with his relentless service returns, made just one unforced error and only dropped serve once — when serving for the match at 5-2 in the second — en route to the biggest win of his career.

The win put De Minaur into a quarter-final match against US qualifier Michael Mmoh, who upset No 8 seed Mischa Zverev 6-2 5-7 6-4.

Raonic was playing his first match since October after taking time out to deal with injuries that also ruled him out of the US Open late last year, when his ranking slipped to 24 — 21 spots lower than his career-high finish in 2016.

Raonic saved double break points in his first two service games in the second set but was finally broken in the fifth game when De Minaur challenged a call on a second serve, got it overturned to win the point on a double-fault and won the next two points on the back of some impressive returns.

De Minaur broke again in the seventh game and, after missing the chance to close out in the eighth, the former Wimbledon junior finalist held his composure to close out.

"It's going to take a while for me to believe what's happened just now," he said.

Asked how he unsettled one of the better servers in tennis, De Minaur said he threw out the playbook.

"I was just guessing really, I'm not going to lie," he said. "I tried to change my returning position a bit to try to throw him off a bit. He probably didn't have his best serving day, which helped me a lot. I took advantage of it, really proud."