Medvedev became only the fourth player in ATP history to beat the world's top three at the same tournament.
Sinner will face 19-year-old Learner Tien for the title.
Stanislas Wawrinka will hang up his racquet at the end of 2026, the three-times Grand Slam champion announced on Friday, bringing down the curtain on a 24-year professional career that saw him gatecrash tennis's elite during the sport's golden era.
Cousins Valentin Vacherot and Arthur Rinderknech will meet in the Shanghai Masters final after pulling off stunning upsets of former champions Novak Djokovic and Daniil Medvedev in the semifinals on Saturday.
Ronaldo got involved in ball kid duties while watching world number one Novak Djokovic's opening match against John Isner of America as the tennis ball bounced into the stands near the first row where he was sitting.
Muguruza wins all-Spanish clash with Badosa at WTA Finals
The Indo-Japanese wild card pair displayed remarkable grit to upset Harrison and King 4-6, 6-3, 18-16 in a match that kept the crowd on the edge of their seats.
Niki Poonacha earns Grand Slam debut, wins Australian Open wild card play-off with Thai partner
Roger Federer got back on track at the ATP Finals with a 7-6(2), 6-3 defeat of Italian Matteo Berrettini on Tuesday.
The 36-year-old was given a thorough workout by Sock although had he been a little more clinical the scoreline would have been more heavily-weighted in his favour.
Only once in 16 previous appearances has Federer not survived the group stage, back in 2008. IMAGES from the ATP Finals played at The O2 Arena in London on Sunday
The event comes at a time when tennis is facing criticism over its relentless tour schedule.
Not that cash will be the motivator for world number one Novak Djokovic, Rafa Nadal, Dominic Thiem, Tsitsipas, Daniil Medvedev, Alexander Zverev and debutants Andrey Rublev and Diego Schwartzman when the tournament's 50th edition starts on Sunday.
MERTENS OVERPOWERS RUSE TO WIN FIRST GRASS TITLE
Grigor Dimitrov claimed the biggest title of his career and sent out a huge statement of intent as the stylish Bulgarian beat David Goffin 7-5, 4-6, 6-3 to win the ATP Finals.
Jannik Sinner beat defending champion and Spanish second seed Carlos Alcaraz to win his first Wimbledon title.
Novak Djokovic felt unaffected by his recent losses to the top two players in the world, Carlos Alcaraz and Jannik Sinner, saying that while it is not ideal to lose big matches to the top talent regularly
Tsitsipas had lost all five previous matches against the Russian, but turned the tables in impressive fashion.
Having dug in early on to stay with Federer, Nishikori ended up running away with the match to inflict a first straight sets defeat in the ATP Finals round-robin stages on the 20-time Grand Slam champion.
Russian Medvedev, 29, last won a title at the Italian Open in May 2023 and has lost six finals since, including at the U.S. Open and Australian Open.
World number one Novak Djokovic began his quest for a sixth title at the season-ending ATP Finals with a 7-6(4), 6-2 victory over Norway's Casper Ruud in their round robin encounter in Turin on Monday.
The 20-year-old German, ranked three in the world, was hot and cold against Croatia's Marin Cilic but, when it mattered most, he produced his best tennis to win 6-4, 3-6, 6-4.
The victory saw him move up by three places to stand only behind Spain's Rafael Nadal and Swiss star Roger Federer.
He also overtook his career-best 65-season win tally of 2023 with this win.
Alexander Zverev became the first German to reach the last four at the ATP Finals for 15 years as he beat American John Isner 7-6(5), 6-3 at the O2 Arena on Friday.
Roger Federer rediscovered his verve in the nick of time to outclass Austrian Dominic Thiem 6-2, 6-3, on Tuesday, and maintain his hopes of reaching the last four at the ATP Finals.
The 45-year-old Bopanna's final appearance on the Tour came at the Paris Masters, where he partnered Kazakhstan's Alexander Bublik, losing in the opening round earlier this week.
Jannik Sinner had to dig deep to get the better of Auger-Aliassime in the second semi-final.
Top seed and defending champion Jannik Sinner, playing on his 24th birthday, ended French qualifier Terence Atmane's dream run at the Cincinnati Open with a 7-6(4), 6-2 win on Saturday to reach the final of the US Open tune-up event.
Monegasque Valentin Vacherot completed a fairytale run at the Shanghai Masters on Sunday, rallying from a set down to defeat his cousin, Frenchman Arthur Rinderknech, 4-6, 6-3, 6-3 in the final and secure his first ATP 1000 title.
Britain's Cameron Norrie stunned world number one Carlos Alcaraz to reach the third round of the Paris Masters.
Sinner's retirement and world number one Alcaraz's withdrawal due to an ankle injury blows the draw wide open, with Djokovic in the hunt for a record-extending fifth title in Shanghai.
Roger Federer has secured his place in the ATP Finals for a record 15th time after the Swiss world number three won a record eighth Wimbledon title on Sunday, the Association of Tennis Professionals confirmed on Monday.
Defending champion Jannik Sinner swept past Felix Auger-Aliassime 6-0, 6-2 on Thursday to reach the Cincinnati Open semi-finals, the world number one extending his hardcourt winning streak to 25 matches.
Novak Djokovic says that defending his Olympic title at LA 2028
'I have no idea what's happening right now, I'm not in a dream, it's just crazy. I'm just so happy with my performance these past two weeks.'
Zverev and Shelton tested each other in a high-quality encounter before the world number three played a near flawless tiebreak to clinch the match in little over two hours and stretch his career win-loss record over the American to 3-0.
It was the 31-year-old Serb's second comfortable win in the Gustavo Kuerten group and combined with Marin Cilic's 6-7(2), 6-3, 6-4 defeat of American John Isner later in the day, he sealed his place in the semi-finals at the year-ender for the eighth time.
Japan Open: Alcaraz marches into quarterfinals, hails "best physio in the world" after overcoming ankle issues
The 18-year-old captured the Argentina Open title with a in over home favourite Francisco Cerundolo in the final.
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