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.
ATP Finals: Alcaraz 'surprised' himself with battling Rublev victory
Carlos Alcaraz surprised himself with the level of performance he was able to produce in his win over Andrey Rublev at the ATP Finals on Tuesday.
Alcaraz emerged a 6-3 7-6 (10-8) victor against the Russian in his second match of the tournament, keeping his hopes of advancing to the next round in Turin alive.
The Spaniard fell to defeat in his opening game against Casper Ruud, citing a stomach issue that left him feeling unwell before and during that match.
But the world number three looked back to his brilliant best against Rublev, claiming the first set in 37 minutes before edging a thrilling tie-break in the second.
"Yes, I surprised myself a lot," Alcaraz said shortly after the victory.
"The way I played today from the baseline, I was really calm and tried to just focus on my game and what I had to do - forget that I'm not really feeling well.
"Once you step on the court, you have to forget everything you're struggling with outside the court."
Alcaraz, though, said he was still not feeling 100% ahead of facing Alexander Zverev, who is in action against Ruud later on Tuesday.
It will be the pair's first meeting since the Spaniard's triumph in the French Open final in June, but Zverev has not let that defeat ruin a fine season.
His own opening win over Rublev was his ATP Tour-leading 67th win of the campaign, with only Jannik Sinner matching the German for victories this year.
"I could be better, healthier. I'm just trying to step on the court and play good tennis, try not to think about my issues right now," Alcaraz said.
"I've been struggling the last week and that's going to be the case over the next day too, for sure.
"Zverev is playing great tennis. He is one of the toughest players in the world right now, that's for sure.
"I will try to play good tennis and enjoy the same way I did today. Then let's see."
ATP Finals: Alcaraz overcomes illness to edge Rublev in Turin
Carlos Alcaraz overcame his recent illness to keep his hopes of progressing at the ATP Finals alive with a straight-sets triumph over Andrey Rublev on Tuesday.
Alcaraz had complained about feeling unwell in his defeat to Casper Ruud, but returned to winning ways with a 6-3 7-6 (10-8) victory in 96 minutes against his Russian opponent.
The Spaniard, sporting a pink nasal strip to aid his breathing, exchanged blows with Rublev early on but was able to take control of the encounter after the fifth game.
Alcaraz reeled off four consecutive games from 3-2 down to seal the opening set, closing in on a second consecutive win against Rublev at the year-end tournament in Turin.
The enthralling affair continued to produce some sensational tennis, most notably when Alcaraz brilliantly turned defence into attack during a long rally in the third game.
The four-time grand slam champion later kept his cool in the second-set tie-break, saving two set points before closing out the win to kickstart his campaign.
Alcaraz now sits at the top of the John Newcombe Group ahead of Alexander Zverev's meeting with Ruud later on Tuesday, with the German up next for the world number three.
Data Debrief: Winning the best medicine
After a rocky start to his ATP Finals campaign, Alcaraz's latest showing confirmed that, despite not feeling at his best, he can still pose a threat at the tournament.
The Spaniard struck 31 winners compared to Rublev's 14, but did match his opponent for unforced errors during the contest (20 each).
But it was another victory on a favoured surface for the world number three. Since the ATP rankings were first published in 1973, Alcaraz (21 years and 192 days) is the second-youngest player to claim 20 top-10 wins on hard courts, older only than Lleyton Hewitt.
ATP Finals: Sinner happy to come through 'very tough match' against Fritz
Jannik Sinner insisted he was prepared for the "aggressive" Taylor Fritz in their first meeting since the US Open final in September at the ATP Finals on Tuesday.
Sinner improved his record over Fritz to 3-1 with a 6-4 6-4 victory in Turin, going level with Alexander Zverev with a Tour-leading 67th victory of the season.
The Italian now sits at the top of the Ilie Nastase Group after two games, and faces Daniil Medvedev on Thursday, with the Russian beating Alex de Minaur in straight sets.
"It was a very tough match. We got to know each other from a grand slam final," Sinner said.
"We knew exactly what to expect today. He was very aggressive, but I was prepared too. I just tried to serve well in the important moments.
"In the return positions, his quality of tennis was very, very high, so I was struggling to return his serve, but I was happy with the situation.
"It was a very important day for me today, and I'm very happy that I got through."
Sinner fought off tough resistance from Fritz in front of a home crowd in Turin, hitting 22 winners compared to his opponent's 19.
Fritz had initially earned the first break point of the match, but was unable to make it count before fighting back from 0-40 to level the first set at 4-4.
Sinner ultimately piled the pressure on to win the first set and capitalised on a double fault in the second to break Fritz and win. Fritz will now face de Minaur on Thursday.
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