The seedings were announced on Wednesday for the Grand Slam tournament that starts Monday, with top-ranked defending champion Novak Djokovic heading the men's list and Maria Sharapova leading the women's — both for the first time.
Hard-hitting Canadian Milos Raonic is seeded 21st.
The women's list follows the WTA rankings, while the men's has minor changes to reflect a player's prowess on grass.
Williams, who played in eight Wimbledon finals from 2000-09, is unseeded for the first time since her debut at the All England Club in 1997. She has dropped to 55th in the rankings while dealing with Sjogren's syndrome, an autoimmune disease that can cause fatigue and joint pain.
Williams has played only five tournaments this year, last appearing in the French Open, where she bowed out in the second round. The last time she won Wimbledon was in 2008. Sister Serena, a four-time Wimbledon champion, is seeded sixth in line with her WTA ranking.
Clijsters, ranked 53rd, is unseeded for the first time since 2000. The Belgian has never reached the final at Wimbledon.
Since her Australian Open title defence ended in the semifinals in January, Clijsters has been limited by ankle and hip injuries. She has played only in the Key Biscayne Masters in March, and this week's Unicef Open, where she has reached the quarterfinals.
Twice a Wimbledon semifinalist, Clijsters intends to retire after the U.S. Open.
The most notable unseeded man was former champion Lleyton Hewitt who, because of a ranking of 202, received a wild card for the 10th anniversary of his victory at the All England Club. He hasn't won consecutive matches this year since the Australian Open.
Andy Roddick, a three-time Wimbledon finalist, was ranked outside an automatic seeding but was lifted by the club to No. 30.
Djokovic is followed in the men's seedings by two-time winner Rafael Nadal, six-time winner Roger Federer and Andy Murray. The rest of the top 10 is rounded out by Jo-Wilfried Tsonga, Tomas Berdych, David Ferrer, Janko Tipsarevic, Juan Martin del Potro and Mardy Fish.
Fish was promoted two places above his ranking to a 10th seeding, one above fellow American John Isner, despite not having played since late March because of a heart problem.
Also promoted for good grass results were Spain's Feliciano Lopez (14), Croatia's Marin Cilic (16), Italy's Andreas Seppi (23) and Mikhail Youzhny (26). Bernard Tomic of Australia, a quarterfinalist last year, was bumped up seven places to 20th.
Sharapova was seeded only 13th when she won Wimbledon in 2004. She was runner-up last year to Petra Kvitova, who will be fourth this year. Australian Open champion Victoria Azarenka is seeded No. 2 and Agnieszka Radwanska No. 3.