From 2007 to 2010, Zecchini attended in rallies with the national team, although she could not compete in the top category due to her young age. In 2011, she participated in her first Italian championship in
Turin, taking second place, behind Ilaria Bonin. The placings achieved also in the following year earned her the call to be part of the national team. In 2012, she participated in her first
Confédération Mondiale des Activités Subaquatiques (CMAS) European Championship At the Italian Depth Championship in
Ischia, she achieved her first CMAS world record in constant weight with the monofin (CWT) with . In 2014, she participated in her first
AIDA International competition, the Team World Championship in Cagliari, winning three silver medals in dynamic apnea with the monofin with , in static apnea with the time of 6' 39", in constant weight with a depth of and setting two Italian records. At the CMAS European Zecchini won the competition in dynamics without fins (DNF) with , also setting the CMAS world record. In that same competition, she also won two silvers (DYN with and STA with a time of 6'35"). During the Outdoor World Championship in Ischia, organised by the Italian Federation of Sport Fishing and Underwater Activities, she won three gold medals with relative world records: in constant weight with monofin (CWT) with , in constant weight without tools (CNF) with and in jump blue (JB) with . Once again, in June 2016 in Lignano Sabbiadoro, during the CMAS Indoor World Championships: three golds and three CMAS world records with in dynamics without fins, in dynamics with fins (DYNBP) and in dynamics with monofin (DYN). In 2017, the Italian began her outdoor training with international coach and safety diver
Stephen Keenan and beat the AIDA world record with depth in the Vertical Blue competition on 10 May on
Long Island in the
Bahamas. During the last days of the competition,
Natalia Molchanova's record in constant weight (CWT) of (which had been held for six years), was beaten three times. On 6 May, Zecchini was the first to go down and take off the tag at ; breaking the world record. Four days later, Hanako Hirose dropped to but minutes later she went on to detach the tag at , setting another world record.
Stephen Keenan died in July 2017 during a recreational dive across
The Arch of the Blue Hole with Alessia. It was not regarded as a dive that would push Zecchini to her limits, but an accumulation of mistakes and difficulties during the attempt would lead to disaster. Safety diver Lily Crespy, who was on duty that day, later gave a detailed summary of events. on stand-by at the dive site. However, for reasons unknown, he decided to dive in 20 seconds later than pre-planned, to be seen by her with the ascending rope at the exit of the tunnel. This caused Alessia, already successfully outside the tunnel and with breath to spare, to swim astray in search of her way up. When Stephen spotted Alessia, he strived to help her to re-surface, giving his own life for hers. In 2017 she also participated in the CMAS European Indoor Championship in
Cagliari swimming pool where she won three medals out of the three competitions (first place in DYN, second in DNF and second in DYNBP). In August of the same year, she participated in the AIDA World Championships in Roatan where she became world champion in free-immersion setting a new Italian record at and vice world champion in constant weight with monofin with . A disqualification denied her the silver medal even in constant weight without fins. In October, at the CMAS European Championship in
Kaş, Turkey, she won two silver medals in CWT and CNF with and , a new Italian record. Also in the same year, during the Dungoncup in
Milazzo, Sicily, she achieved the world record in dynamic apnea without fins with . Zecchini then came under the guidance of Martin Zajac, which lasted until the conclusion of the 2018 competition season. In 2018, Zecchini set the new AIDA world record in constant weight (CWT) on 10 May in
San Andrès in Colombia in the Nirvana Oceanquest Freediving Competition. A month later, at the Indoor World Championships in
Lignano Sabbiadoro, Italy, she won two silver medals in DYN and DNF and a gold medal in the bi-fin discipline where she set the new world record with . In July, in Long Island, in the prestigious Vertical Blue competition, she confirmed herself as the strongest freediver by winning overall and setting four world records AIDA in all depth disciplines: – Constant weight without fins with , Free immersion with and then and Constant weight with monofin with . In September of the same year, she participated in the Dugoncup Outdoor in Milazzo setting two other world records in CWT () and FIM () but this time under the aegis of CMAS, before flying to the first edition of the Molchanova Gran Prix in memory of Natalia Molchanova, winning the competition. She participated in the CMAS Outdoor World Championship by winning three golds in the three competitions held, CWT, CNF and FIM where she set four CMAS world records, bringing the WR in constant weight with CMAS and AIDA to the same depth of . In April 2019, during the Dungoncup in Milazzo, she again set the world record in dynamic free diving without fins with . In June 2019, she participated, together with the Italian National team, in the CMAS Indoor
European Championship in
Istanbul, winning two gold medals with relative world records, in dynamics with fins and with monofin, and a silver medal in dynamics without fins. On 7 August 2019, during the CMAS Outdoor World Championship in
Roatan, Honduras, at a depth of , she set the new world record in constant weight freediving with monofin at the same time as Slovenian
Alenka Artnik. During the Nirvana Oceanquest 2019 event in Curaçao, she set two world records in the discipline of free immersion under the aegis of AIDA with on 16 October 2019. Two days later, with approved by CMAS, she became the first woman to touch this depth using only her arms. In October, she took part in the CMAS Outdoor World Championship 2021, in
Kaş, Turkey, winning a gold medal in CNF and one silver medal in CWTB. During the "CMAS TSSF
Kaş Baska Freediving WC (competition before the world championship), she set a new world record in her first competition in CWTB with the depth of , more than the previous record. In October 2022, during the CMAS Outdoor World Championship 2021 in Kaş, she won a silver medal with in CWT. On 27th March 2023, during the Secretblue Competition in
Moalboal, Philippines, she set a new world record in CWTB with . After two days, she went deeper and set a new world record with in 3'36". On 24 May 2023 she became again the deepest woman in the world, setting the WR in constant weight with monofin to -123m, during the AIDA Oceanquest Competition in Camotes Island, Philippines. ==Honors==