[5] In 2004, he won the Pakistan Open title in Islamabad (upsetting Amr Shabana in the quarter-finals) in his first appearance in a PSA Super Series final.

[6] In 2005, he finished runner-up at the British Open as the seventh seed, then followed this by lifting the Qatar Classic trophy in only his second Super Series final appearance.

[4] In the 2006 Commonwealth Games in Melbourne, Willstrop partnered with Vicky Botwright to a silver medal in the mixed doubles.

[9] In 2007, Willstrop won the British National Squash Championships title, beating John White in the final,[10] helped England retain the 2007 Men's World Team Squash Championships title in Chennai, India, and won the English Open, beating fellow Yorkshireman Nick Matthew in the final.

[11] Willstrop retained his British National title in February 2008, beating fellow Pontefract player Lee Beachill in the final.

[12] In 2009, he reached the semi finals of the 2009 Men's World Open Squash Championship and in January 2010, Willstrop won his first Tournament of Champions title in New York, defeating World Number 1 Ramy Ashour in the final and dropping only one game during the tournament.

[17][16] In 2019 and 2020, Willstrop won the British National title, beating Daryl Selby and Joel Makin respectively in the finals.