WATCH: Coco Gauff, Frances Tiafoe are tested on Serena Williams trivia in ATP challenge

Who is the better Serena Williams (center) fan? Coco Gauff (left) and Frances Tiafoe (right) take on the ATP Tour's challenge.

Coco Gauff and Frances Tiafoe figured in a battle for the most knowledgable Serena Williams fan in the ATP Tour's Head-to-Head Serena Challenge.

Tiafoe, who confessed to being a big fan of the 23-time Grand Slam champion at the start of the video, gained an early lead in the quiz and went on to edge Gauff, 8-7.

That led to Gauff jokingly telling Tiafoe that he just got lucky.

"Frances (Tiafoe), you got lucky, but I know more about Serena (Williams) than you do," said Coco Gauff.

The questions ranged from personal ones – Williams' birthday and middle name – to some history about her tennis career – her age when she started playing professionally, when she won the US Open for the first time, and when she broke Steffi Graf's Grand Slam record.

Other tidbits about Williams were also included, such as the number of her Grand Slam and singles career titles she has won, the number of Major finals she contested against older sister Venus Williams, her mixed doubles partner at the 2019 Wimbledon Championships and others questions.

Both Coco Gauff and Frances Tiafoe started strong, getting Williams' 23 Grand Slam titles correctly. But both did not know Williams' birthday, each citing how Serena did not celebrate hers (Williams is following the practice of her religion, Jehovah's Witness).

Tiafoe knew that Serena started her professional career at 14, but Gauff guessed 15, thus pushing the ATP World No. 24 2-1 up.

Both shared three more incorrect answers, but Gauff and Tiafoe guessed several other questions correctly – Serena lifting the US Open trophy for the first time in 1999, Jameka as her middle name, and learning and speaking French at an awarding ceremony among others.

Coco Gauff, Frances Tiafoe among participants in Tennis Plays for Peace

Coco Gauff (right) and John McEnroe (left)

Coco Gauff and Frances Tiafoe shared their talents for a worthy cause – in the US Open's exhibition event for humanitarian relief for war-struck Ukraine.

Gauff partnered with legend John McEnroe as they kicked off the Tennis Plays for Peace charity doubles match together with opponents and eventual winners Rafael Nadal and Iga Swiatek.

Tiafoe was paired with Carlos Alcaraz, battling against Taylor Fritz and Tommy Paul in the fourth match. Fritz and Paul won through a sudden-death point at 9-all.

Tiafoe immediately returned to wrap up the night's matches – held on Ukraine's Independence Day – teaming up with the embattled nation's Dayana Yastremska, who entered the court with her country's flag draped around her shoulders. They faced American Sebastian Korda and Daria Snigur of Ukraine.

The other matches pitted Ben Shelton and Jessica Pegula against Felix Auger-Aliassime and Leylah Fernandez, and Italy's Matteo Berrettini and Ukraine's Katarina Zavatska versus Greeks Stefanos Tsitsipas and Maria Sakkari.