[3] Tien started playing tennis when he was just a toddler, and won his first tournament at the Racquet Club of Irvine at the age of 5.

In doubles compiling a 36–18 win-loss record and winning the 2023 Australian Open, partnering Cooper Williams.

[7] In July, Tien also received a wildcard for the 2024 Cranbrook Tennis Classic in Bloomfield Hills, Michigan where he lifted his maiden Challenger title.

[8] He became the youngest American Challenger champion since 2016, when an 18-year-old Frances Tiafoe won in Granby, Canada.

[13] He lost to Yunchaokete Bu, ending a 28-match winning streak across ITF and ATP Challenger tournaments.

He became the youngest American ATP tour-level quarterfinalist since Brandon Nakashima in 2020 (in Delray Beach).

[19] He won his second Challenger title in Las Vegas, defeating Tristan Boyer,[20] and moved up another 40 positions in the singles rankings to a new career-high of No.

He became the fourth American to win three ATP Challenger Tour titles before his 19th birthday after Taylor Fritz, Andy Roddick and Sam Querrey.

[29][30][31] Tien defeated Corentin Moutet in straight sets to reach the fourth round of a Grand Slam for the first time in his career and moved into the top 100 in the singles rankings.