Top seed Dominic Thiem was beaten in straight sets in the second round of the Dubai Tennis Championships by Lloyd Harris.
The reigning US Open champion, given a bye for the first round, lost 6-3 6-4 to his South African opponent, who sits 77 places further down the ATP rankings.
The Austrian has endured a difficult start to 2021. He lost in straight sets to Grigor Dimitrov in the last 16 of the Australian Open and was beaten by Roberto Bautista Agut in the quarter-finals in Doha.
It was a first win over a top-five player for Harris, who was not once taken to deuce on serve during their 72 minutes on court.
"I'm super, super happy with that win. [He is] by far the highest-ranked player I have beat," said Harris. "I look up to Domi a lot so, for me, it is a special feeling. I am really happy. Just the fact that I am playing good tennis, match in, match out now is giving me a lot of confidence.
"I was serving really well. I don't think I faced any break points. I took control of those games and [in] one or two games on his serve, I gave myself a good look. I played some of the important points very well. I am very proud of that."
While Thiem is struggling for form, things are looking far better for Andrey Rublev, who beat Emil Ruusuvuori 6-4 6-4 to stretch his winning streak at ATP 500 events to 21 matches.
The world number eight, who will face Taylor Fritz in the next round, equalled Andy Murray for the second-best winning run at this level of the Tour. Roger Federer (28) holds the record.
Third seed Denis Shapovalov beat Jan-Lennard Struff for the loss of just four games, while Bautista-Agut advanced after opponent Matthew Ebden retired in the first set.
Fifth seed David Goffin was beaten in straight sets by Kei Nishikori, who recorded just his third victory of the year.
Karen Khachanov and Jannik Sinner each advanced after three-setters, while Marton Fucsovics and Aslan Karatsev were also taken the distance before progressing.
Hubert Hurkacz, Lorenzo Sonego, Dusan Lajovic, Aljaz Bedene and Filip Krajinovic also won on Tuesday.
Sinner guides Italy to back-to-back Davis Cup triumphs
Jannik Sinner capped his outstanding year by helping Italy retain the Davis Cup, beating Tallon Griekspoor to ensure Italy downed the Netherlands 2-0 in Sunday's final.
World number one Sinner overcame Griekspoor 7-6 (7-2) 6-2 after Matteo Berrettini had registered a 6-4 6-2 win over Botic van de Zandschulp in the opener.
Sinner saved the only two break points of the first set as Griekspoor put up a fight, but the Italian put on a show in the tie-break to inch ahead.
Griekspoor hit back after giving up an early break in the second set, but Sinner immediately responded with two further breaks, with the Dutchman double-faulting for the first then wildly miscuing with a backhand for the second.
Sinner was untroubled from then on, sealing a triumphant end to a spectacular season.
The Italian has only dropped one set throughout that streak, finishing a year that saw him win the Australian Open, US Open and ATP Finals with a 73-6 singles record.
Data Debrief: Sensational Sinner leads the way
This has been a truly remarkable year for Sinner, who has become the first man in the Open Era to win at least one set in every match throughout a season since Roger Federer in 2005.
His victory ensured Italy became the first team to retain the Davis Cup since the Czech Republic triumphed in 2012 and 2013.
Italy also captured the Billie Jean King Cup earlier this week, and they are just the third nation to claim both trophies in the same season in the last 30 years, after the Czech Republic in 2012 and Russia in 2021.
Sinner and Berrettini send defending champions Italy into Davis Cup final
Jannik Sinner's victory over Alex de Minaur clinched Italy's place in another Davis Cup final, with Matteo Berrettini also victorious as the defending champions beat Australia 2-0.
Italy won the Davis Cup for the first time since 1976 last year, and they will have an opportunity to defend their crown against the Netherlands on Sunday.
Former Wimbledon finalist Berrettini got the ball rolling with an entertaining 6-7 (6-8) 6-3 7-5 victory over Thanasi Kokkinakis, not surrendering a break after losing the opener in a tie-break.
That teed up Sinner to close out Italy's victory, with the world number one overcoming De Minaur 6-3 6-4, the same scoreline he beat the Australian by at the ATP Finals earlier this month.
Speaking after his victory in Malaga, Sinner said: "It means a lot. It was a tough encounter, playing Alex. We know each other quite well now, so I have to be very careful every time.
"It's a very tough challenge for me, so I was looking forward to this one. Obviously, it helped a lot that Matteo won the singles today. He played amazing tennis. It was very high quality.
"Hopefully, this can give us some confidence for [Sunday]. It's going to be a very difficult and tough day for us and also the Netherlands."
Data Debrief: Back-to-back for Italy
Italy's men are looking to follow in the footsteps of their female counterparts, who won the Billie Jean King Cup earlier this week.
They are just the third nation in history to make back-to-back finals at both the Davis Cup and the Federation/Billie Jean King Cup, after the United States (1963-64, 1978-79, 1981-82 and 1990-91) and Australia (1963-64 and 1964-65).
Djokovic appoints Murray as coach for Australian Open
Novak Djokovic has announced his long-term former rival Andy Murray as his coach for the Australian Open in 2025.
Murray, a three-time Grand Slam winner, announced his retirement earlier this year and played his final match at the Paris Olympics in August.
He will join Djokovic's team in the off-season and coach him through the opening grand slam of the year, which begins on January 12.
This will be Murray's first coaching role since retiring following a 19-year career.
"We played each other since we were boys. 25 years of being rivals, of pushing each other beyond our limits," Djokovic said in a video on social media.
"We had some of the most epic battles in our sport. They called us game changers, risk-takers, history makers.
"I thought our story may be over - turns out it has one final chapter.
"It's time for one of my toughest opponents to step into my corner. Welcome onboard coach, Andy Murray."
Djokovic failed to win a major title for the first time since 2017, though he did clinch a long-awaited Olympic gold medal in Paris.
The Serb is currently level with Margaret Court for the most major singles titles by any player. He has a good record in Australia though, winning 10 times in Melbourne, beating Murray in four of those finals.
Murray, who beat Djokovic to win the US Open title in 2012 and Wimbledon the following year, says he hopes they can achieve success together.
"I'm going to be joining Novak's team in the off-season, helping him to prepare for the Australian Open," Murray said.
"I'm really excited for it and looking forward to spending time on the same side of the net as Novak for a change, helping him to achieve his goals."
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