Peterborough can choose their opponents in the semi-finals by virtue of topping the league table and the two sides left will face each other.
Comets team manager Tony Jackson expects to find out who they will face by the end of today.
He said: "Friday was a tremendous result but now we just have to wait and see.
"The main thing was getting to the play-offs, although it would have been nice to finish top to have our pick of the sides."
Comets suffered a 49-41 defeat in their final away league meeting of the season at Newcastle on Sunday, but top spot in the SGB Championship was already out of reach after Peterborough beat Lakeside at Rye House the previous night.
Workington were without their top three, with Nicolai Klindt riding in Poland and Ty Proctor and Dan Bewley injured, and they were almost without Rene Bach as the Dane was feeling unwell.
Jackson said: "With just over three hours to go before start time, Rene got in touch to say that he had been very unwell and didn’t think he would be able to ride.
"At that stage, we had nowhere else to go so simply told him to get to the track and see how he felt and, if he couldn’t do it, then just pull out as our options to replace him were absolutely zero!
"When we saw him, it was clear that he was ill but we persuaded him to ride."
Aaron Summers, guesting at No.1 for Klindt, had an inspired night to score 19 points from seven rides, while fellow Berwick and former Comet Kevin Doolan did not fare as well as a guest for Bewley, only managing two points and a bonus from five rides.
Workington made the better start with a 4-2 from the guest duo in the opening heat but that was wiped out moments later by a home 5-1.
The hosts stretched their lead to eight points over the next four heats but Comets took a chunk out of that with a 5-1 of their own in heat seven from Bach and Summers.
That was the visitors’ last heat advantage until the final race, with the Diamonds steadily increasing their lead over the second half of the meeting as Workington struggled to match their opponents.
Workington will host the Diamonds at Derwent Park in a Knockout Cup quarter-final second leg on Friday (7.30pm start) before a Championship Shield semi-final second leg at home against Scunthorpe on Saturday (7pm start).
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