Diab began competing for the Illinois Fighting Illini men's gymnastics in 2016. During his freshman year, he won the still rings title at the 2016 Big Ten Championships, with a score of 15.125. That was the eighth event title he won during the season, tying for second-most on the team. He also earned All-America honors on the horizontal bar at the NCAA Championships, finishing fifth on the horizontal bar with a score of 14.900. Following the season he was named Fighting Illini Newcomer of the Year.[2]
During his sophomore year, he won his second-straight Big Ten still rings event title, with a score of 14.875.[3] He was NCAA All-American on still rings, earning a score of 14.650 for seventh place overall at the NCAA Championships.[4] In August 2017, he competed at the 2017 World University Games.[5]
2018
In February 2018, he competed at the 2018 Winter Cup, posting a combined score of 28.550 on still rings and won silver.[6]
During his junior year in April 2018, he competed at the NCAA Championships, where he posted a score of 14.500 on still rings and won gold and helped his team win bronze at the event. The win marked his 10th rings title during the season, which is the most in a season in program history.[7]
In August 2018, he competed at the 2018 U.S. National Gymnastics Championships, where he posted a score of 14.500 on still rings on day one, and 14.450 on day two, for a combined score of 28.950, and won silver.[8]
2019
In February 2019, he competed at the 2019 Winter Cup, where he posted a score of 14.550 on still rings during the preliminary round, and 14.600 to finish with a two-day total of 29.150 and won silver in the event. He also placed 10th on both floor and vault, finishing with scores of 27.250 and 28.350, respectively.[9]
On March 2, 2019, Diab set a career-best and Illinois program record score of 15.200 on rings. He was subsequently named Big Ten Gymnast of the Week.[10] During his senior year in April 2019, he competed at the NCAA Men's Gymnastics Championships, where he won his second straight national championship on still rings with a score of 14.733, becoming the first gymnast in program history to win two national titles on the event. Diab finished his Illinois career as a six-time NCAA All-American, a four-time Big Ten still rings champion, a two-time still rings national champion, the school's all-time leader with 31 titles on still rings.[11] Following the season, he was named a finalist for the Nissen Award and 2019 CGA Specialist of the Year.[12][13]
In February 2020, he competed at the 2020 Winter Cup, where he posted a score of 14.600 on still rings in the preliminary round, and scored a 14.850 to finish with a two-day total of 29.450 to win gold in the event. He also competed in floor exercise, vault, and horizontal bar, placing within the top 20 on each apparatus.[15]