Saints made the perfect start, taking a ninth-minute lead through Shane Long, who finished off a Bertrand cross from close range after Pierre-Emile Højbjerg's flick on.
But back came Liverpool, as Naby Keita equalised late in the first half before Mo Salah struck the killer second ten minutes from time, and Jordan Henderson wrapped up the points with the visitors' third.
“It's very hard [to take]," said Bertrand. “I think we worked really well, but you've got to do it for 90 minutes.
“The disappointing thing is that we gave them their goals. They were avoidable, but on the other hand you can't say we deserved anything after giving them goals."
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