2017–2018

As a freshman, Diakomihalis won the Bearcat Open, New York State Intercollegiate, and the Cliff Keen Invitational and compiled a record of 28-1 during the regular season.^[1] Post-regular-season, he won the EIWA championships and later went on to win the NCAA championship, joining Kyle Dake as the only Big Reds to do so in their "true freshman" year of competition and earning notable victories over #1 Bryce Meredith, #2 Jaydin Eierman, and two-time NCAA champion Dean Heil.^[9][10] Diakomihalis suffered a torn ACL during his quarterfinal matchup against #6 Heil.^[11]

2018–2019

As a sophomore, Diakomihalis won the Mat Town Open I and the South Beach Individual and capped a perfect 13–0 record in dual matches, ending the regular season unbeaten.^[12] He went on to win his second EIWA championship and earned the Outstanding Wrestler award in the process.^[13] At the NCAA championships, he most notably defeated the #2, #3, and #5 ranked wrestlers in Jaydin Eierman, Joseph McKenna and Dominick Demas to claim the national title.^[14] Diakomihalis was named EIWA Top Wrestler and was a finalist for the Dan Hodge Trophy.^[15]

2019–2020

Diakomihalis did not compete at the NCAA level in 2019–2020, as he took an Olympic redshirt year and focused on freestyle.^[16]

2020–2021

Yianni planned to return to college wrestling in 2020–2021; however, the Ivy League announced the cancellation of all winter sports on November 13, 2020, due to the COVID-19 pandemic.^[17]

2021–2022

After 975 days, Diakomihalis finally returned to the mat for the Big Red on November 20, 2021, defeating Stanford's Jaden Abas at 149 pounds. He finished 28-0 and won his third NCAA title (his first at 149 pounds).^[18]

2022–2023

Diakomihalis finished his college career with his fourth title at the NCAA tournament in Tulsa (his second title at 149 pounds) with a 4-2 victory over Sammy Sasso of Ohio State University. Diakomihalis became the fifth wrestler in NCAA history, and the second from Cornell University, to win four NCAA titles.

2019

After opting for the Olympic redshirt, Diakomihalis attended his first senior level tournament at the US Open in April. He went 6–0 (three technical falls) with notable wins over Frank Molinaro, Jordan Oliver and Zain Retherford in the finals.^[21] Not long after, he competed in an exhibition match at the Beat the Streets charity event against multiple World medalist Bajrang Punia. He won the match on points (10–8).^[22]

After winning the US Open championship, Diakomihalis sat out of the World Team Trials Challenge, having earned himself an automatic spot at Final X: Rutgers with the US Open victory; US Open finalist Retherford won the World Team Trials Challenge to earn the other spot at Final X in June.^[23] In the first match of the best-of three series, Diakomihalis fell 10-4. The second match ended controversially. Retherford was down 4–6 late in the match when he got a takedown for two points, while Diakomihalis was awarded two points for a back exposure to apparently win the match 8-6, as time expired while the wrestlers were still grappling. Retherford's corner challenged the points awarded to Diakomihalis and won the challenge, resulting in a final score of 6–6 with criteria advantage to Retherford, giving him a 2-0 lead in the series and an apparent victory at Final X.^[24] However, subsequent to the tournament, Diakomihalis initiated an arbitration challenging the scoring change. Diakomihalis ultimately prevailed in the arbitration with a finding that the scoring review was conducted improperly.^[25] The arbitrator awarded Diakomihalis a rematch of the second match in the series.

In July, Diakomihalis competed at the prestigious Yasar Dogu Grand Prix in Istanbul, Turkey. He defeated four opponents such as his Final X opponent Zain Retherford, Ali Rahimzade and Ismail Musukaev to claim the championship.^[26]

Two weeks later he went on to compete in the Waclaw Ziolkowski Memorial at Warsaw, Poland. He defeated four opponents (two technical falls) including Ismail Musukaev (by forfeit) to claim the championship.^[27]

In September, the rematch between Retherford and Diakomihalis took place in a special event called Final X: Yianni vs. Zain in Wilkes-Barre, Pennsylvania. Diakomihalis lost the bout on points, 2-1, losing his chance to represent the United States at the World Championships.^[28]

In December, he competed at the US Nationals. He defeated three opponents before getting upset by Joseph McKenna in the semifinals. He advanced to the finals of the consolation brackets but forfeited his final match.^[29]

2020

In his first tournament of the year, Diakomihalis competed at the Pan American Championships. He defeated four opponents including Agustín Destribats and NCAA Division II champion Jose Rodriguez to claim the championship.^[30]

Diakomihalis was scheduled to compete at the US Olympic Team Trials on April 4–5 at State College, Pennsylvania.^[29] However, the event and the Olympics were postponed due to the COVID-19 pandemic, leaving him and all the qualifiers unable to compete.^[31]

After more than half a year without stepping on the mats, Diakomihalis wrestled at the annual Beat The Streets event against the heavily accomplished Vladimer Khinchegashvili on September 17.^[32] He lost the competitive and close match by criteria.^[33]

Diakomihalis represented the Spartan Combat Wrestling Club at 65 kilograms in the FloWrestling: RTC Cup on December 4–5.^[34] He tech'd 2020 Big Ten Championship runner-up Sammy Sasso (Ohio State) in the first round, followed up with two criteria victories over 2017 World Championship runner-up James Green (New Jersey) and got a final TF over 2018 NCAA All-American Tariq Wilson to help the team reach the third-place.^[35]

2021

To start off the year, Diakomihalis wrestled two-time and reigning Pan American champion (70 kg) Anthony Ashnault on January 8, at the SCRTC I.^[36] After scoring two two-point takedowns and a step-out, he scored a five-point slam, driving Ashnault to a technical fall.^[37] Diakomihalis then competed at the Henri Deglane Grand Prix of France on January 16.^[38] After advancing to the semifinals with two wins, Diakomihalis faced World and Olympic champion Vladimer Khinchegashvili in a rematch from their BTS match, and was able to avenge his lone 2020 loss when he dominantly tech'd the Georgian, advancing to the finals, where he flawlessly defeated James Green, in a rubber match from their series at the RTC Cup, to claim gold.^[39]

Diakomihalis then competed at the America's Cup on February, defeating DI All-Americans Mitch McKee (twice) and Pat Lugo to help the Team Kenny Monday reach sixth place.^[40] On March, he defeated Matt Kolodzik in a dual match against the NJRTC.^[41]

In April 2–3, he competed at the rescheduled US Olympic Team Trials as the second seed, in an attempt to represent the United States at the 2020 Summer Olympics.^[29] After defeating the defending US Olympic Team Member Frank Molinaro, he was upset by third-seeded and 2019 US National champion Jordan Oliver, ending Diakomihalis' win streak.^[42] He came back to wrestle for third–place after defeating Mitch McKee, but was once again defeated, now by 2021 NCAA champion Nick Lee, placing fourth.^[43]

After his loss at the Olympic Trials, Diakomihalis came back at the prestigious Poland Open, where he was the defending champion, on June 9.^[44] In the second round, he was knocked off by Iszmail Muszukajev in a frenetic rematch from their also frenetic match in 2019.^[45] Next, he most notably dominated recently crowned Pan American Continental champion Joseph McKenna to a technical fall and defeated the same opponent he defeated in his first match, 2018 University World Champion Eduard Grigorew, winning the championship without getting the rubber match with Musukaev as Musukaev forfeited out of the tournament after their earlier match.^[46]

Diakomihalis then competed at the 2021 US World Team Trials from September 11 to 12, intending to represent the country at the World Championships.^[47] After dropping All-American Luke Pletcher to make the finale, Diakomihalis faced Joseph McKenna in a best-of-three match.^[48] After losing a razor-close first bout, he won the next two, including a dominant technical fall in order to become the 2021 US World Team member.^[49]

Diakomihalis represented the United States at the 2021 World Championships on October 3 in Oslo, Norway.^[50] After a dominant first round win over Italy, he was eliminated by 2020 Individual World Cup winner Vazgen Tevanyan on points.^[51]

2022

In May, at the 2022 World Team Trials Challenge tournament, Diakomihalis defeated Luke Pletcher and Ian Parker, both by 10-0 technical superiority, to advance to Final X against Evan Henderson. ^[52]

At Final X in June at Madison Square Garden, Diakomihalis defeated Henderson 11-9 and 14-4 to win the best-of-three series and earn his second straight spot on Team USA for the World Championships.^[53]

At the 2022 World Championships in Belgrade, Diakomihalis made history as the first 65 kg US wrestler to earn a medal at the World Wrestling Championships since Bill Zadick in 2006. He advanced to the finals with three straight wins. He began with a 4-0 win over Vazgen Tevanyan of Armenia, avenging his loss at the 2021 World Championships. He followed that with a 10-0 win by technical superiority over Vladimir Dubov of Bulgaria. In the semifinals, he defeated Bajrang Punia of India by technical superiority, 10-0 to advance to the finals.^[54] His semifinal victory guaranteed him a silver medal and clinched the team title for the United States.^[55] In the finals, Diakomihalis was defeated by Rahman Amouzad of Iran by a score of 13-8, thereby claiming the silver medal.^[56]

2023

In May 2023, Diakomihalis represented Team USA at the Pan American Championships, going 4-0 and earning the gold medal.^[57]

His silver medal at 2022 Worlds entitled Diakomihalis to skip the preliminary qualifying rounds for Team USA and advance directly to Final X.^[58] At Final X, he lost consecutive matches to Nick Lee, 7-6 and 8-8 (criteria).^[59]