The German sixth seed, who has not won a title since winning at Flushing Meadows last year, never got into the match against the 2016 WTA newcomer of the year.
American teenage sensation Coco Gauff became the youngest player to win a WTA title in 15 years as she beat former French Open champion Jelena Ostapenko in the Linz Open final on Sunday.
The exodus of seeded players has been so spectacular that only one of the top 10 remains -- number seven Karolina Pliskova.
World number two and top seed Caroline Wozniacki, runner-up at the south coast event last year, cruised into the semi-finals with a 6-4, 6-3 win over Australia's Ashleigh Barty.
'It's very emotional. I've been waiting for this moment since I was 14'
Tactical change helps Halep to French Open semis
Next up for the 35-year-old will be another American in 39th-ranked Tommy Paul with a potential semi-final clash against top-seeded Russian Daniil Medvedev, who he defeated in the Australian Open final from a two-sets deficit.
IMAGES from Day 7 at Wimbledon on Monday
He leads the men's seedings ahead of defending champ Roger Federer, with Bulgarian Grigor Dimitrov seeded third.
Images from Day 6 of Wimbledon at the All England Lawn Tennis and Croquet Club in London, on Saturday.
Caroline Wozniacki moved a step closer to erasing an unwanted footnote from the list of her tennis achievements with victory over Svetlana Kuznetsova in the French Open fourth round on Sunday.
US Open champion Sloane Stephens said she has no pity for Madison Keys after thrashing her 6-3, 6-0 in Saturday's final, given the huge runnersup check her friend will receive.
Former Wimbledon runner-up Eugenie Bouchard successfully negotiated the qualifying draw on Thursday, beating Colombian Mariana Duque-Marino 6-3, 6-2 to secure a spot in next week's main draw.
Less than a month after her run to the Wimbledon final, where she lost to Spain's Garbine Muguruza, 37-year-old Williams again had to turn back time against a young challenger.
Millman, ranked 80 places above world number 14 Pouille, needed 73 minutes to clinch a 6-3, 6-4 victory and the best win of his career by ranking position.
Novak Djokovic enjoyed a stress-free return to US competition following a two year absence
Three-times winner Serena Williams moved into the Toronto Open second round after beating Spain's Nuria Parrizas Diaz on Monday
Germany's Angelique Kerber, who will try to prevent Serena Williams winning an eighth Wimbledon title on Saturday, never went away but she too is making a comeback of sorts.
Newly-crowned French Open champion Simon Halep said she was inspired to win her first Grand Slam title by her manager Virginia Ruzici's feats 40 years ago at Roland Garros.
Simona Halep will take over as the new number one in the WTA world rankings after progressing to the China Open final with a dominant 6-2 6-4 victory over Latvia's Jelena Ostapenko in Beijing on Saturday.
Serena Williams will be 'ready to go' when she returns to action this week in the United States' Fed Cup first round tie against the Netherlands, captain Kathy Rinaldi said on Wednesday.
Serena Williams reluctantly pulled out of this year's Australian Open on Friday, the American saying that while she was close to regaining full fitness after giving birth to her first child in September, she was not fully ready to defend her title.
The Slovakian eventually won 6-4, 6-1 but the match will be remembered for a ludicrous umpiring decision at a crucial stage of a tight opening set.
Djokovic is clubbed with Federer, Juan Martin Del Potro, David Goffin and Alexander Zverev.
Images from matches played on Day 5 at the Australian Open in Melbourne Park on Friday
Earlier, Ukraine took a 2-0 lead over Japan as world number five Elina Svitolina took the first point of the Billie Jean King Cup era.
Serena, a six-times champion in New York who is seeded 17th, will start her campaign against Polish world number 60 Magda Linette and could face Venus in the third round before a possible fourth-round showdown with Romanian top seed Halep.
The year's final grand slam began with an upset as 13th-seeded Frenchman Richard Gasquet was upended 6-2 6-2 6-3 by Britain's world number 84 Kyle Edmund at the U.S. Open on Monday. Gasquet, who has reached the last 16 at Flushing Meadows four times in his career, was outplayed throughout and his U.S. Open experience lasted just one hour, 41 minutes.
'I will put everything I have Saturday on the court'
Former men's champion Mats Wilander believes third seed Muguruza now looks like the most likely champion, having not dropped a set and thrashing Maria Sharapova on Wednesday.
'Maria will look at it and see no one dominant. Serena Williams is the only player in the past who has been like a thorn in her side, but she feels mentally superior to most of the other and she feels she can win against them all'
Maria Sharapova was dogged by injury and controversy as she tried to get her career back on track.
Konta, Sharapova fall at Rogers Cup in Toronto
Serena was drawn on Friday to play doubles in a potentially decisive fifth rubber in this week's Fed Cup first round tie against the Netherlands in what will be her first competitive match in over a year. The doubles rubber, which is scheduled for Sunday on an indoor hard court in Asheville, North Carolina, will be played even if one side clinches the best-of-five match series through the opening four singles matches.
Simona Halep's brave Australian Open campaign but the exhausted Romanian hit a dead end in the final as she crumbled in a thriller against Caroline Wozniacki.
Simona Halep and Caroline Wozniacki will compete for high stakes in Saturday's Australian Open final, having each needed to save match points to prevent a premature exit at Melbourne Park.
French favourite Kristina Mladenovic skidded out of a storm-struck French Open on Tuesday, losing 6-4, 6-4 to Timea Bacsinszky in a rain-interrupted quarter-final in Paris.
As another Grand Slam crown looks set to be snaffled by a man in his thirties this weekend, tennis fans lamenting a dearth of dynamic up-and-comers need not worry, says former world number one Mats Wilander.
Images from Day 4 of the Australian Open in Melbourne on Thursday
Images from Day 6 of the Australian Open in Melbourne, on Saturday.