section_path: Professional career/2021: Four titles, Masters final, top 10
section_name: 2021: Four titles, Masters final, top 10
paragraph_text: 2021: Four titles, Masters final, top 10
  
  Sinner carried over his success from late 2020 into the start of the 2021 season. He won his second career ATP title at the Great Ocean Road Open, and notably defeated No. 20 Karen Khachanov in the semifinals after saving a match point. He became the youngest to win back-to-back ATP titles since Rafael Nadal in 2005. His ten-match winning streak came to an end in the first round of the 2021 Australian Open, where he lost a tight five-set match to world No. 12 Denis Shapovalov.
  
  Sinner's next big result was at the Miami Open, where he reached his first ATP Masters 1000 final. During the tournament, he defeated Khachanov again and later world No. 12 Roberto Bautista Agut in the semifinal. He finished runner-up to Hubert Hurkacz.
  
  Then at the French Open, his campaign was stopped short for the second year running by Rafael Nadal, who this time defeated Sinner in straight sets in the fourth round. In his main draw debut at Wimbledon, he lost in the first round to Márton Fucsovics.
  
  Partnering Reilly Opelka he won his first doubles title at the 2021 Atlanta Open, defeating Steve Johnson and Jordan Thompson. At the same tournament in singles he fell in the second round to Christopher O'Connell.
  
  At the 2021 Citi Open in Washington, D.C., Sinner went into the tournament as the fifth seed and made it to the finals and beat several young players along the way such as Emil Ruusuvuori, Sebastian Korda, and Jenson Brooksby. He beat Mackenzie McDonald in the final to win his third title and first ATP 500 title. Sinner was the first Italian finalist and champion in Washington's tournament history as well as the youngest ATP 500 and first teen champion since the category was created in 2009. As a result, he entered the top 15 in the ATP rankings on 9 August 2021.
  
  At the US Open, he defeated Gaël Monfils in the third round to reach the second week of a Major for the second time in the season. Sinner's tournament ended when he lost to Alexander Zverev in the 4th round in straight sets.
  
  Sinner successfully defended his title at the Sofia Open as the top seed, defeating again second seed Gaël Monfils in the final. Sinner made his sixth career final at the 2021 European Open without dropping a set en route. He defeated Lorenzo Musetti, Arthur Rinderknech and Lloyd Harris to reach the final. He bested Diego Schwartzman in the final to take his fifth career title. He became the youngest man to win five ATP titles since 19-year-old Novak Djokovic.
  
  On 1 November, Sinner became the first male player born in the 2000s to break into the top-10 after a semifinal appearance at the Vienna Open. At the Rolex Paris Masters, Sinner received a bye in the first round but was defeated by Carlos Alcaraz. Because of this, Sinner was unable to directly qualify for the season-ending Nitto ATP Finals.
  
  At the ATP Finals in Turin, Sinner was present as the first alternate. Sinner entered the tournament after countryman Matteo Berrettini was forced to withdraw with an abdominal injury after his first match with Alexander Zverev. He defeated Hubert Hurkacz and became the youngest player to win an ATP Finals match on debut since Lleyton Hewitt in Lisbon in 2000 and the first alternate to win a match since Janko Tipsarević in London in 2011. Sinner played Daniil Medvedev next in the round robin stage, holding a match point before being defeated in 3 sets. As a result, he re-entered the top-10 in the rankings and finished the year at world No. 10 on 22 November 2021.
  
  In the Davis Cup Finals, Sinner defeated John Isner becoming only the second player (after Thiemo de Bakker) ever to bagel (6–0 set win) Isner.
  
  Sinner carried over his success from late 2020 into the start of the 2021 season. He won his second career ATP title at the Great Ocean Road Open, and notably defeated No. 20 Karen Khachanov in the semifinals after saving a match point. He became the youngest to win back-to-back ATP titles since Rafael Nadal in 2005. His ten-match winning streak came to an end in the first round of the 2021 Australian Open, where he lost a tight five-set match to world No. 12 Denis Shapovalov.
  
  Sinner's next big result was at the Miami Open, where he reached his first ATP Masters 1000 final. During the tournament, he defeated Khachanov again and later world No. 12 Roberto Bautista Agut in the semifinal. He finished runner-up to Hubert Hurkacz.
  
  Then at the French Open, his campaign was stopped short for the second year running by Rafael Nadal, who this time defeated Sinner in straight sets in the fourth round. In his main draw debut at Wimbledon, he lost in the first round to Márton Fucsovics.
  
  Partnering Reilly Opelka he won his first doubles title at the 2021 Atlanta Open, defeating Steve Johnson and Jordan Thompson. At the same tournament in singles he fell in the second round to Christopher O'Connell.
  
  At the 2021 Citi Open in Washington, D.C., Sinner went into the tournament as the fifth seed and made it to the finals and beat several young players along the way such as Emil Ruusuvuori, Sebastian Korda, and Jenson Brooksby. He beat Mackenzie McDonald in the final to win his third title and first ATP 500 title. Sinner was the first Italian finalist and champion in Washington's tournament history as well as the youngest ATP 500 and first teen champion since the category was created in 2009. As a result, he entered the top 15 in the ATP rankings on 9 August 2021.
  
  At the US Open, he defeated Gaël Monfils in the third round to reach the second week of a Major for the second time in the season. Sinner's tournament ended when he lost to Alexander Zverev in the 4th round in straight sets.
  
  Sinner successfully defended his title at the Sofia Open as the top seed, defeating again second seed Gaël Monfils in the final. Sinner made his sixth career final at the 2021 European Open without dropping a set en route. He defeated Lorenzo Musetti, Arthur Rinderknech and Lloyd Harris to reach the final. He bested Diego Schwartzman in the final to take his fifth career title. He became the youngest man to win five ATP titles since 19-year-old Novak Djokovic.
  
  On 1 November, Sinner became the first male player born in the 2000s to break into the top-10 after a semifinal appearance at the Vienna Open. At the Rolex Paris Masters, Sinner received a bye in the first round but was defeated by Carlos Alcaraz. Because of this, Sinner was unable to directly qualify for the season-ending Nitto ATP Finals.
  
  At the ATP Finals in Turin, Sinner was present as the first alternate. Sinner entered the tournament after countryman Matteo Berrettini was forced to withdraw with an abdominal injury after his first match with Alexander Zverev. He defeated Hubert Hurkacz and became the youngest player to win an ATP Finals match on debut since Lleyton Hewitt in Lisbon in 2000 and the first alternate to win a match since Janko Tipsarević in London in 2011. Sinner played Daniil Medvedev next in the round robin stage, holding a match point before being defeated in 3 sets. As a result, he re-entered the top-10 in the rankings and finished the year at world No. 10 on 22 November 2021.
  
  In the Davis Cup Finals, Sinner defeated John Isner becoming only the second player (after Thiemo de Bakker) ever to bagel (6–0 set win) Isner.