Willis made his ATP tour debut at the 2016 Wimbledon Championships after qualifying for the main draw, where he gained recognition after playing against Roger Federer in the second round on Centre Court.

[5] Willis was sent home from the 2008 Australian Open by the Lawn Tennis Association (LTA) for his "slack attitude" when he missed the bus to a practice session and eventually arrived without rackets, having left them at his hotel.

He won three matches to qualify for the main draw of the 2016 Wimbledon Championships, which included wins against future top-ten players Daniil Medvedev and Andrey Rublev.

[10] In the first round, Willis upset Ričardas Berankis, a player ranked more than 700 places above him, to set up a clash with seven-time champion Roger Federer.

[14] Injury prevented Willis from playing immediately after Wimbledon 2016, but he was given a wild card into qualifying for the Erste Bank Open 500 in Vienna over the weekend of October 22–23.

He has also been invited to play Tie Break Tens at the same venue on Sunday 23 October, against Andy Murray, Dominic Thiem and Jo Wilfried Tsonga.

[18] They caused a big surprise by defeating the defending champions and second seeds Nicolas Mahut and Pierre-Hugues Herbert in the next round, also in a five-setter.

[22] After Wimbledon, he played for the San Diego Aviators in the 2018 World TeamTennis league,[23] where the team narrowly missed the WTT finals.

[27] In January, Willis won his second doubles title on the ATP Challenger Tour in Oeiras, partnering Jay Clarke.

Over the course of the next five months he went on to win another five Challenger titles including his first on grass at the Nottingham Open, partnering John Peers to a three set victory in the final.

[29] In October he won his seventh challenger title of the season at the Taipei OEC Open, partnering David Stevenson to a straight sets victory.