With Walsh Jennings playing possessed at the net and May-Treanor covering ground in the back end, the two-time defending Olympic champions reeled off six points in a row. They eventually swept the younger, quicker Chinese duo in straight sets.
That means their final match together will be Wednesday's final against fellow Americans April Ross and Jen Kessy. Whether they are on the podium with gold or silver - bet on gold - it will be the end of an astonishing run.
It's like a farewell concert by Simon and Garfunkel, the last film for Laurel and Hardy, one final sandwich made with peanut butter and jelly.
"It's an emotional thing," Walsh Jennings said. "We have one more match together. We're not thinking about that yet, but it heightens everything."
Walsh Jennings, 33, will continue playing. She will find a new partner and do her best to be in Rio de Janeiro for the 2016 Olympics. It is May-Treanor, 35, who has pulled the plug. One win from a third gold medal, she was having exactly zero second thoughts.
"This is it," May-Treanor said. "It's the right time for me. I know Rio is in her sights. If I wasn't ready, it would be emotional, but I'm ready. I'm ready."
They are in different places, and that was evident throughout the match. As with any great relationship, whether a volleyball team or a friendship or a marriage, the partners take turns picking up the slack for each other.
Here, it looked as if May-Treanor was playing pretty well because she still can, while Walsh Jennings was playing great because she's still driven to do so.
It was Walsh Jennings who knocked over a photographer diving to save a ball out of bounds in that first-set comeback. And it was Walsh Jennings who made a physics-challenging shot from under the net that put the Chinese team on the brink. And it was Walsh Jennings who slammed home the final point.
"We were holding on too tight, because we did want it too bad," Walsh Jennings said. "And that stifled us, instead of allowing us to play inspired and free."
The woman known as "Six Feet of Sunshine" shone brightly on a dreary, drizzly London evening - just as she had done in the sizzle of Athens and the broil of Beijing.
"Always," Walsh Jennings said. "I'm an inspired woman. I have a lot to be grateful for. I have a lot of faith in my partner and myself. I'm always inspired. We're in the Olympics. I'm playing for my entire country and my family and for this woman, who I love with all my heart."
They played traditional volleyball against each other in college. They became partners in 2001, after May (pre-Matt Treanor) and another partner finished fifth in the Sydney Olympics. They arrived in Athens as the hot young team. They have won 20 Olympic matches in a row. It will take a 21st to lock up three gold medals.
They are the classic duo. Walsh Jennings is a 6-footer, blond, as outgoing as the sun is bright. May-Treanor is shorter, brunette, more introspective and occasionally even downcast.
When she says that Walsh Jennings is relentlessly positive - "every day, she does that every day" - you can almost sense that it's a challenge for her to answer in kind.
Walsh Jennings has two sons with her husband, fellow volleyball player Casey Jennings. The younger is named Sundance, and it's easy to imagine Walsh Jennings as the freewheeling Sundance Kid to May-Treanor's more complicated Butch Cassidy.
And now they are side-by-side, about to make their last stand together.
"We're out here for a reason," May-Treanor said, "and that's to stand together at the top of the podium."
The whole Bolivian army doesn't know what it's in for.
Contact Phil Sheridan at 215-854-2844 or email@example.com. Follow on Twitter @Sheridanscribe. Read his blog, "Philabuster," at www.philly.com/philabuster, and his columns at www.philly.com/philsheridan