Australia's Hewitt earned the accolade for the second year running, while American Williams was honoured for the first time.
Both players enjoyed outstanding seasons, with the 21-year-old Hewitt winning the men's singles at Wimbledon for the first time and Williams dominating the women's tour by taking three of the four grand slam titles.
Adelaide-born Hewitt, who won five titles in 2002, capped his year with a flourish by securing the season-ending world number one spot with victory at the Masters Cup in Shanghai.
Despite competing in only 13 tournaments, Williams won eight titles
If the 21-year-old American wins the 2003 Australian Open in January, she will become the fifth female player to hold all four majors at the same time.
Having taken over the number one ranking after reaching July's Wimbledon final, Serena rapidly increased her lead at the top of the standings and ended the year 940 points ahead of her nearest rival, sister Venus.
The ITF also named Mark Knowles of the Bahamas and Canada's Daniel Nestor as the men's doubles world champions.
Paola Suarez and Virginia Ruano Pascual were given the women's doubles world champions award.
