Winning start: Joao Fonseca of Brazil

Jeddah (Saudi Arabia) (AFP) - Brazilian teenager Joao Fonseca stunned top seed and 2023 runner-up Arthur Fils with a five-set victory in his NextGen ATP Finals opener on Wednesday.

The 18-year-old Fonseca, the youngest player in the eight-man field, made light of his lowly 145 world ranking to defeat his 20th-ranked French opponent 3-4 (⁹⁄₁₁), 4-2, 4-1, 1-4, 4-1.

Fils arrived in Jeddah as the title favourite having been runner-up to Hamad Medjedovic in 2023, before winning two tour titles on the ATP Tour this season.

However, Fonseca had defeated Fils at Rio in February for his maiden tour-level win.

Elsewhere in the Blue Group on Wednesday, 122nd-ranked Learner Tien of the United States upset Czech world number 48 Jakub Mensik 4-3 (⁸⁄₆), 4-3 (⁷⁄₃), 2-4, 2-4, 4-3 (¹⁰⁄₈).

Fonseca will face Tien in his next match on Thursday having defeated the American in the final of the US Open junior final last year.

“It was a good match. I knew it was going to be hard from the first point,” said Fonseca, who secured his first Top 20 win.

“I also knew that the victory that I had against him in Rio would be part of this match. He also knows what I’m capable of… I think in important moments, I played better. I went to the ball and played aggressive like I always play. I think that was the difference today.”

Tien secured his five-set win after saving a match point against Mensik, who clubbed 20 aces in the match.

“It was tough playing a guy that serves that well,” said Tien.

France’s Luca Van Assche, a semi-finalist in 2023, overcame Shang Juncheng of China 4-3 (⁷⁄₃), 2-4, 4-1, 4-3 (⁷⁄₅) in the Red Group.

“I think the experience of last year helped me a lot today, because I already played with these rules,” said 128th-ranked Van Assche.

“For ‘Jerry’, it was his first time, so it’s difficult to be prepared to play sets of four games and all these rules.”

Shang, ranked at 50 in the world, can still make the semi-finals with American pair Alex Michelsen and Nishesh Basavareddy also in his pool.

World number 41 Michelsen defeated Basavareddy in four sets in his opener.