Defending ATP Finals champion Daniil Medvedev maintained his perfect start to this year's tournament but found it hard going against Alexander Zverev, eventually winning 6-3, 6-7 (3), 7-6 (6) in their Red Group clash on Tuesday.
Medvedev extended his run of wins in the tournament to seven and is already guaranteed a semi-final place having beaten Polish debutant Hubert Hurkacz in his opening match.
Zverev's opening opponent Italian Matteo Berrettini retired injured and withdrew from the tournament on Tuesday.
Berrettini's place was taken by fellow Italian Jannick Sinner who gave the home fans plenty to cheer in the evening session with a 6-2 6-2 defeat of Poland's Hurkacz.
Russian world number two Medvedev, who won his first Grand Slam this year at the US Open, raced into a 3-0 lead in the first set by frequently coming to the net and looked like he would take a comfortable fifth straight victory over Zverev.
However, the German found some confidence in Turin to bring himself back into the contest, firing down 13 aces and taking the second set on a tiebreak.
The pair could not be separated in the decider with neither able to break serve and both soaking up some heavy hitting.
In the decisive tiebreak, Zverev saved two match points but could only hit a backhand into the net on a third to give Medvedev victory after two hours and 35 minutes.
"Definitely one of the matches to remember," Medvedev said. "When you win 8-6 in the tiebreak in the third. Not actually much to say about the match, just amazing."
Zverev, 24, faces Poland's Hurkacz in his final round-robin match looking to clinch a place in the last four.
The 20-year-old Sinner can also still reach the semi-finals after his impressive demolition of Hurkacz.
"I knew around 5pm that I would play," Sinner said. "I was ready to go on court. It is an incredible feeling playing here in Italy with thousands of people cheering for you.
"I will enjoy the moment. I played well and felt good in the warm up. It was an incredible match today."
The Green Group resumes on Wednesday with Novak Djokovic facing Andrey Rublev and Stefanos Tsitsipas taking on Casper Ruud.
Berrettini announced that he was withdrawing from the event in a post on Instagram after suffering an abdominal injury.
"My finals end here, I'm destroyed, I never thought I'd have to give up the most important tennis event ever held in Italy in this way," he wrote.
"The truth is that as much as I wanted to play in front of you once again, I felt and therefore decided that my body is not ready to face the challenges that lie ahead."
Sinner said he felt bad for Berrettini.
"Matteo has been unlucky many, many times in his career and I wish him all the best," Sinner said. "I am going to play for him at this tournament because he deserves to be here."
Muguruza wins all-Spanish clash with Badosa at WTA Finals
Garbine Muguruza advanced to the championship match of the WTA Finals with a 6-3 6-3 win over fellow Spaniard Paula Badosa in Guadalajara, Mexico on Monday.
In their first career meeting, Muguruza's superior firepower put Badosa on the back foot from the outset and she broke the Indian Wells champion for a second time with a forehand winner to take the first set.
Badosa's frustration boiled over during the change over and she was unable to hold serve in the lengthy second game of the second set, giving the former world number one and two-time Grand Slam champion another early break.
Muguruza fended off Badosa's three break point chances in an 11-minute third game to take a 3-0 lead and rolled on from there, firing an ace to go up 5-2 and sealing the win when Badosa hit a backhand into the net on match point.
"I think it's the best match that I played so far here in Guadalajara," Muguruza said.
"It was a tough match facing another Spaniard in the semifinals of a Masters. Actually the first time we've encountered together ... so it was tricky.
"But I'm very happy that I got the win."
Muguruza, who will end the year ranked world number three, will face either Maria Sakkari of Greece or Estonian Anett Kontaveit, who play later on Tuesday, in Wednesday's final.
The loss ends a breakthrough year for Badosa, who will finish the season at career-best world number eight.
"Today was a tough day," Badosa said.
"I didn't feel myself. I didn't play the match that I was expecting."
The season-ending WTA Finals were not held last year because of the COVID-19 pandemic and were originally scheduled to take place in Shenzhen before being moved because of pandemic-related travel restrictions in China.
