Serena Williams' husband Alexis Ohanian proudly displays a PSA Card of his wife from 1999 ahead of her final tournament 

Reddit founder Alexis Ohanian, husband of 23-time Grand Slam winner Serena Williams, revealed that he is a big collector of Professional Sports Authenticator (PSA) cards.

PSA is the largest and most trusted third-party trading card authentication and grading company in the world.

Ohanian stated that there are only eight PSA 10 (the highest grade on the scale) Serena Williams cards in the world and that he owns four of them. The card is from 1999 when Williams won her first Major at the US Open.

"There are only 8 of these in the world (graded PSA 10) and I own 4 of them (all vaulted safely at @altxyzofficial of course)," Ohanian wrote on Twitter.

Ohanian also said that he would like to own more PSA 10 cards in the future, especially one in particular that featured both the Williams sisters.

"Yes there's one really dope 2003 netpro that has one sister on each side," he wrote.

Ohanian and Williams have been married since 2017 and have a four-year-old daughter, Olympia.

Serena Williams training hard ahead of the US Open

Rennae Stubbs talks to Serena Williams during a practice session in New York

Serena Williams was seen training hard with new coach Rennae Stubbs ahead of her first-round match at the US Open against Danka Kovinic on Monday night.

The New York Major will be Williams' final tournament before retiring, and many fans are hoping that she has a fairy-tale run. Williams will be keen on equaling Margaret Court's record of 24 Majors, but her recent form suggests that is unlikely.

The legendary tennis player has played four matches this season, losing three of them. Her most recent loss came against reigning US Open champion Emma Raducanu in Cincinnati last week.

Williams has won the US Open a record six times (along with Chris Evert), but has lost her last two finals at Flushing Meadows in 2018 and 2019.

World No.1 Daniil Medvedev was full of praise for Williams recently, saying that the 23-time Grand Slam champion has a "legendary energy around her."

“Every time I’ve seen her, she has this legendary energy around her that I’m sure is what makes people see her and say, ‘I’m a fan,’ and we will probably still be talking about her for another 30 years," he said.