The World No.1 and defending champion crushed Germany's Annika Beck 6-3 6-0 to move into second place in the all-time list -- just six wins behind Martina Navratilova's Open era record of 306.
Williams, who will play Russia's Svetlana Kuznetsova in the next round, has won 82 matches at Wimbledon and is vying not only for her seventh Wimbledon triumph but also a 22nd grand slam which would take her level with Open era record holder Steffi Graf.
"Oh, was it? Cool. Oh, nice. I had no idea. That's awesome, right? I think that's a lot of matches. It's a lot of wins," Williams told reporters after being informed of her achievement.
"I thought it was good. I still want to get out to a little bit of a faster start but I was really focused and calm.
"Obviously I love having that Sunday off but I'm a little behind in my matches so I guess I had to play an extra day."
This was just the fourth time in Wimbledon's 130-year history that tennis has been played on the middle Sunday -- the last coming 12 years ago in 2004.
Some 22,000 tickets went on sale to the general public on Saturday after organizers decided that the backlog of matches which had been postponed by the bad weather which had played havoc with the opening week of the tournament.
Those that got in were treated to a gripping contest between France's Jo-Wilfried Tsonga and America's John Isner.
It was Tsonga who eventually prevailed by winning 19-17 in the fifth set with some fans fearing the match could go on all night.
That's because six years ago Isner was involved in the longest tennis match in history, beating Nicolas Mahut 70-68 in the fifth set.
Tsonga will now play compatriot Richard Gasquet who defeated Spain's Albert Ramos-Vinolas in four sets.
Australia's Nick Kyrgios set up a fourth round tie against Andy Murray after overcoming Spain's 22nd seed Feliciano Lopez.
Kyrgios, seeded 15, won 6-3 6-7 6-3 6-4, but has lost four times to Murray on tour although he did defeat the Scot at the Hopman Cup earlier this year.
"I definitely feel like he's beatable. He's only human. At the same time he's a great player," Kyrgios told reporters.
"This is his backyard. He's won here before. But this is probably my best surface, my best chance to beat him.
"At the same time he's pretty comfortable on the grass."
Del Potro downed
Juan Martin del Potro's run at Wimbledon came to an end Sunday after he went down in four sets to France's 32nd seed Lucas Pouille.
He was joined by German teenager Alexander Zverev, who was beaten in straight sets by Czech 10th seed Tomas Berdych.