US Open Day 8 ended in stunning fashion as Roger Federer became the biggest casualty of the tournament thus far after the world number two was knocked out by Australian John Millman in the round of 16.

The Swiss tennis legend looked off color for most of the game and struggled to keep pace with the Australian’s aggressive approach in the sweltering heat in Arthur Ashe Stadium.

It was the first time the five-time US Open champion lost to a player ranked below 50 in the world at Flushing Meadows. Federer had a bad day in office in terms of service, he committed 10 double faults — with two of them coming during the fourth set tiebreaker.

The Swiss ace looked comfortable after winning the opening set 6-4 in 33 minutes and had two set points in the second serving 5-4, but the Australian broke Federer in consecutive games to win the second 7-5 and turn the tide.

The number two seed was again ahead in the third set leading 4-2, when Millman struck again to break back and level the game at 4-4. The set then went into a tie-break which the world number 55 won.

The fourth set also went into a tie-break, but that was a no contest as the Australian flew into a 6-1 lead. Millman converted his third match point to create the biggest upset of the tournament to set up a clash with Novak Djokovic in the quarter-finals.

Federer was favorite going into the match with most already talking about a potential clash between the number two seed and Djokovic, who had made his way into the last eight with a straight sets win over Portugal’s Joao Sousa earlier in the day.

However, Millman had other ideas and the Australian will now take on the Serbian, who is one of the pre-tournament favorites. And Federer’s draw will open out the bottom half of the draw that also has Marin Cilic and Kei Nishikori, who face each other in the other quarter-final.

“Today, he [Federer] was definitely not at his best but I’ll take it,” Millman said after his win. “I felt like a bit of a deer in headlights to begin with, the feet weren’t moving, Roger had it on a string and he was manipulating me around the court.”

“I got out of a tough second set and really found my feet and started to be a bit more aggressive. I started to serve well and capitalised on Roger having an off service day,” the Australian added.

Meanwhile, in the women’s draw, Maria Sharapova was knocked out by Carla Suarez Navarro, who  celebrated her 30th birthday Sunday. The Russian, who looked to be getting back to her best after a dominating win over Jelena Ostapenko in the third round went down in straight sets to the Spaniard.

Navarro sets up a quarter-final clash with 2017 finalist Madison Keys, who beat Dominika Cibulkova 6-1, 6-3 in round four on Sunday. 21-year-old Naomi Osaka also made it to the last eight and will take on Caroline Wozniacki’s conqueror Lesia Tsurenko.