But the 36-year-old wasn't dozing on court Tuesday at the Australian Open, as the American battled past Anastasia Pavlyuchenkova 6-4 7-6 (7-3) at Rod Laver Arena to become the oldest women's semifinalist in the Open Era of the tournament.
Witt was full of admiration for Williams, who in 2011 was diagnosed with Sjogren's syndrome, an autoimmune disorder that causes, among other things, fatigue and joint pain.
Not ideal for anyone, athletes included.
"It's crazy to think how long she has played," Witt told CNN as Williams cooled down in the player gym. "And she's still doing it at a high level. It's incredible.
"Part of that comes down to her fight and belief, how much she has in her heart -- every time she goes out there, you never see her quit on a point, never see her tank a game or two, and that says a lot. It takes a lot out of someone to do that.
"Yeah, so it's crazy."
Whose age record did the ever professional Williams surpass? Someone she knows well.
Younger sister Serena, who was 34 when reaching the last four 12 months ago.
And no, Williams isn't thinking about calling it quits anytime soon. Why would she, given she can still compete with the best of them?
"I feel like I have a lot of great tennis in me," she told reporters. "Any time you feel that way, you continue."
With Serena still in contention -- the 22-time grand slam winner meets Britain's Johanna Konta in her quarterfinal on Wednesday -- a first all-Williams grand slam final since 2009 Wimbledon looms.
It almost happened last year at the All England Club but Venus -- bidding for an eighth major and first since 2008 at Wimbledon -- fell in the semifinals to current No. 1 Angelique Kerber.
Thursday's opponent, fellow California native CoCo Vandeweghe, isn't -- on paper -- as prolific. She is, after all, ranked 35th and will be contesting her first grand slam semifinal.
Vandeweghe, though, followed up her emphatic fourth-round victory over Kerber by crushing 2016 French Open winner Garbine Muguruza 7-5 6-0 later Tuesday.
"Once I got rolling in the second (set), it was like a freight train, you couldn't stop it," Vandeweghe said in her on-court interview.
Her record against top-10 opposition since the start of last year -- 6-1 -- is proof of the danger she presents.
Vandeweghe and Williams share a power game where serves often set the tone, but the latter is considerably more understated with words than her 25-year-old compatriot.
Williams opted for the secondary locker room at Melbourne Park, not the "A" locker room where higher-ranked pros habitually reside.
That, Witt says, isn't unusual.
"That's just Venus," he said. "She's about as easy going as it gets. I think that's why we get along so well. What you see is what you get. She's not picky, not high maintenance. She doesn't need to be catered to. She's independent, she does things herself."
Williams defeated Vandeweghe in their lone tussle, last year on clay in Rome, in straight sets.
No sets lost
Speaking of sets, Williams has yet to surrender one at this year's championships, which Witt says has been pivotal in terms of managing her fatigue issues.
"It's part of the reason maybe why she's in the semis," he said. "We've talked about it several times before where she's at the US Open, at certain tournaments, she'll be in the second or third round, maybe she can close a match out in two sets and she goes to three sets, it's a three-hour match and the next thing you know she either injures something or she's tired for the next match.
"That's the importance of getting on and off the court quick. It's all about your body."
Despite not appearing in a grand slam final for eight years -- and an Australian Open final in 14 years -- Williams' winning mindset hasn't wavered.
Asked if she believes she can win the year's first major, her reply was resolute.
"Why shouldn't I?" Williams said. "I try to believe. Should I look across the net and believe the person across the net deserves it more? This mentality is not how champions are made."