Andrey Rublev finally got the better of compatriot Daniil Medvedev after a flashpoint involving a courtside camera in the Western and Southern Open semi-finals.
Rublev will now face Alexander Zverev in the decider, having ended Medvedev's bid for a Toronto-Cincinnati double.
Medvedev had never even dropped a set to his fellow Russian in four prior ATP Tour meetings and appeared to be on course for another dominant victory when he took the first set.
But the world number two clattered into a camera early in the second and all momentum was soon lost.
Medvedev complained about the positioning of the camera, claiming it had caused a hand injury and aiming a kick at the lens.
He swiftly called for treatment as his performance started to fall well below his lofty standards, with Rublev finally able to win a set after breaking in an epic 15-minute game.
A series of unforced Medvedev errors allowed Rublev to break again in the decider and seal a stunning 2-6 6-3 6-3 triumph.
Third seed Zverev fought back from a double break down in the final set to progress to the final with a 6-4 3-6 7-6 (7-4) win over second seed Stefanos Tsitsipas.
The epic match lasted two hours and 41 minutes, with Zverev responding strongly after appearing unwell to book his spot in the final against Rublev.
Rublev gets his Daniil degree
Asked to reflect on finally toppling Medvedev, Rublev told Amazon: "It's always tough to play against Daniil and to beat him.
"I think it gives me a bit more confidence that I can play against him, I can compete against him. There are still so many things to improve, but it's like I've passed university."
The victory came as a relief, with Rublev believing he was unfortunate even to be trailing in the first set.
"Inside I was thinking, when I was 6-2, the score shouldn't be like this," he said.
"The points were really tight, some little outs, little mistakes, some good shots from Daniil. The score was not real [in] the first set.
"Even the third set, I won 6-3 but the match was so intense. You saw so many rallies, so many long rallies, and it was so tough.
"It was a super mental match, a super physical match, exactly like a chess match."
Zverev's Novak mentality
Tokyo 2020 gold medalist Zverev had trailed 4-1 in the third set against Tsitsipas, but fought back with two breaks before winning in a tie-break.
"After I did the first break back I thought 'OK I have the chances'," Zverev said during his on-court interview. "I felt like he was not serving bombs. I felt like I was always in the rallies but I was losing the rallies because I was a bit low energy, so I started being a bit more aggressive, a bit of the Novak mentality that I had against him at the Olympics as well."
Zverev has a 4-0 record against final opponent Rublev but he was wary of his opponent.
"Favourite or not, I think if you're in the final, there's no easy opponent," he said. "Today he played incredible beating Medvedev."
WTA Finals: Sabalenka digs deep to reach semi-finals
Aryna Sabalenka became the first player to book her place in the WTA Finals semi-finals after her straight-sets win over Jasmine Paolini on Monday.
The Belarusian had to dig deep in the second set but prevailed in 91 minutes, beating the Italian 6-3 7-5 in Riyadh to secure top spot in the Purple Group after two games.
While Sabalenka cruised through the first set, she was almost forced to a decider, but saved two set points in the 10th game of the second to level the score at 5-5 before going on to claim the victory.
In doing so, she became the first player to reach back-to-back semi-finals at the tournament as world number one since Serena Williams in 2013-14.
Having won three of her previous four tournaments and won 22 of her 23 matches in that time, Sabalenka is now just one win away from securing the year-end world number one spot for the first time, a feat she can achieve if she beats Elena Rybakina on Wednesday in the final round-robin match.
And she can go into it with confidence after improving her record against top-five opponents in 2024, with this her fifth such triumph.
"I'm proud of myself. Not only myself, my team," Sabalenka said.
"We were able to overcome a lot of things. To be able to show such great tennis and become World No. 1. It's teamwork. It's not only me. The behind-the-scenes work nobody sees.
"But they do a lot for me. I really appreciate them for everything they do for me. This is motivation for me to keep winning on this court. Those guys deserve to be called the best team ever."
In Monday's other match, Zheng Qinwen kept her hopes of reaching the final four alive with her win over Rybakina.
The Olympic champion was forced to go the distance though, despite getting the first break in the first and second sets, but eventually came out on top 7-6 (7-4) 3-6 6-1 in just under two-and-a-half hours.
Since the event's inauguration in 1972, Zheng is just the second Chinese player to win a match at the WTA Finals after Li Na.
It was a milestone victory for the 22-year-old as well as she earned her 50th win of 2024, improving her record to 50-17, making her the first Chinese player in the Open Era to hit that mark in a calendar year, though she acknowledged she could have made things easier for herself.
"I was the one to make the first break, and then I let her back in the first," Zheng said.
"Then it happened again in the second. But in the third set, I talked to myself. I broke her first and I stayed focused.
"Finally, in the third set, I start to play my tennis again. So, I'm really happy to win the match."
WTA Finals: Swiatek battles from the brink to make winning start
Iga Swiatek began her bid to regain the world number one ranking by recovering from a set and double break down to beat Barbora Krejcikova in her first match at the WTA Finals.
Swiatek, playing in her first competitive game since her US Open quarter-final exit, emerged a 4-6 7-5 6-2 victor against the Wimbledon champion in Riyadh.
The Pole is the third player in the last 30 years to achieve three or more seasons in a row with over 60 wins, after Martina Hingis (1997-2001) and Caroline Wozniacki (2009-2011).
Only Elina Svitolina (12) has played fewer matches at the WTA Finals than Swiatek (13) to achieve 10 wins at this event since the reintroduction of the Round Robin format in 2003.
But the Pole acknowledged the difficulties she faced against Krejcikova, saying she must improve if she is to retain her WTA Finals crown.
“Even though I played a lot of those [practice] matches,” Swiatek said afterward. “I kind of forgot for a while how it is to feel all those things, a bit different stress and emotions.
"For sure, I needed some time to adapt. The most important thing was that even though it happened, I managed to fight through that. And was patient enough to wait to get better.”
In Sunday's other match, Coco Gauff beat fellow American and doubles partner Jessica Pegula 6-3 6-2 in the second Orange Group match.
Gauff moved to a win-loss record of 51-16 in 2024, including the Olympics and team events, equalling her career-high from 2023 for WTA-level match wins in a calendar year.
She is now only the fifth player to claim 20 or more WTA top 10 wins before turning 21, along with Kim Clijsters (45), Maria Sharapova (39), Ana Ivanovic (26) and Wozniacki (23) since 2000.
"I think maybe I served better than previous times and this is our first time, I think, playing each other on indoor hard, so maybe that had something to do with it," Gauff said.
"I don't know. I think I know I've lost to her a lot, but it doesn't feel like it sometimes.
"Because we do practice a lot, and obviously, when we're playing sometimes I win practice sets, sometimes she does. So it doesn't feel like the head to head is as bad as it was."
Zverev soars past Humbert to claim Paris Masters crown
Alexander Zverev soared to the Paris Masters title following a commanding 6-2 6-2 victory over home favourite Ugo Humbert in the final.
The German took just 75 minutes to seal his seventh career ATP Masters triumph, and second silverware of the season.
Humbert received a raucous reception as he aimed to become the first Frenchman to win this event since Jo-Wilfried Tsonga in 2008.
However, the crowd were silenced when Zverev broke in the third and fifth games on his way to taking the opening set.
The Italian Open champion also dominated the second set, dropping just five points on serve as he wrapped up a comfortable victory.
"I knew I had to play like this to win today," Zverev said. "Ugo is an incredible player, but here in Paris, he plays even better than he usually does, and I knew that.
"Once the crowd gets involved, it's going to be difficult. So, I had to take that away early, and I did, so I'm happy about that."
Zverev was also delighted to triumph in Paris, and demonstrate the strides he has made since suffering a horrifying ankle injury during his 2022 French Open semi-final against Rafael Nadal.
"It was not 100% guaranteed that I would be back at this level," he added. "So, to win this title here in Paris means the world to me, and I'm sure it means everything to those in my box, because they have done so much for me."
Data Debrief: Zverev surpasses Medvedev and matches Sinner
Runner-up to Daniil Medvedev in 2020, Zverev went one better at the Accor Arena four years later, and will be seeded second at the season-ending Nitto ATP Finals.
The second German to win this event after Boris Becker, he surpassed Medvedev (six) with his seventh ATP Masters crown the outright most of any player born since the format's inception in 1990.
Zverev also matched world number one Jannik Sinner for the joint-most match wins in ATP Masters events this season with 28.
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