The second-seeded Williams broke back twice to stay in the second set, then escaped in the tiebreaker to complete a 6-4, 7-6 (5) victory over 92nd-ranked Jarmila Groth of Australia.
Williams was pushed hard in the match, which lasted 1 hour, 37 minutes on Court 2.
The first set was decided in the 10th game when Williams converted the only break point of the set as Groth netted a backhand volley. The 23-year-old Groth broke to open the second set and twice served for the set at 5-4 and 6-5, but Williams took advantage of break chances each time.
Williams raced to a 4-0 lead in the tiebreaker, but Groth rallied to 4-4 and 5-5. The Australian then doublefaulted to hand Williams a match point, which ended with Groth netting a short forehand.
The tight nature of the match was reflected in the statistics: Williams had 26 winners and 15 unforced errors, while Groth had 25 winners and 16 errors.
The victory sent Williams into her 31st Grand Slam quarterfinal and improved her record to 12-0 in fourth round matches at the All England Club. She remains on track to meet sister Serena, the defending champion and three-time winner, in Saturday's final.
The match got off to a bizarre start, with Venus arriving on court nearly 10 minutes late. As Groth and the chair umpire waited at the net, Williams didn't arrive until 12:09 local time for the scheduled noon match. A stern-faced Williams didn't apologize or speak at all as she went to her chair. Then after only three games, Williams took a restroom break.
The first result of the day came when fourth-seeded Jelena Jankovic retired due to a back injury while trailing Vera Zvonareva 6-1, 3-0. It's the fourth time the Serb has gone out in the fourth round here, while the Russian made it to the quarters for the first time.
Monday's schedule featured all 32 remaining players in action in men's and women's fourth-round matches.
There were two eye-catching matchups on the women's side: Serena Williams against 2004 champion Maria Sharapova, and an all-Belgian showdown between former No. 1s Justine Henin and Kim Clijsters, both making Wimbledon comebacks after returning from retirement.
Six-time champion Roger Federer faced former junior doubles partner Jurgen Melzer of Austria for the first time in the opening Centre Court match. Rafael Nadal, who got through the third round despite a worrisome knee injury, plays Frenchman Paul-Henri Mathieu.
Fourth-seeded Andy Murray, trying to become the first British man to win Wimbledon since 1936, faced big-serving Sam Querrey. The 18th-seeded American is 8-0 on grass this month and won his first grass-court title two weeks ago at Queen's.