The defending champion used a combination of speed and deep ground strokes to force Davydenko into costly errors.
Ferrer won the first set as Davydenko struggled with his serve early, committing four double faults and repeatedly volleying long.
Ferrer broke the former world No. 3 in the fifth game of the second set before saving a late break point to ensure the victory.
"I began well and then later I had some trouble, but I was able to pull it out in the end," said Ferrer. "I feel good. I'm serving pretty well, but I still need to improve my game."
"Tomorrow will be a hard match," Monaco said. "All of us have reached the semifinals in good form."
Granollers, last year's runner-up to Ferrer, saved four set points in the first-set tiebreaker.
Monfils rallied to claim the second set, but Granollers took the match in two hours and 26 minutes when the Frenchman struggled with his serve late and sent his final forehand wide.
"These two years have been great for me here," said Granollers, who won his second ATP title in July. "Monfils is a tough player. He serves well and moves incredibly well. I had to play hard to beat him."
Monfils' early exit means he will have to win the Paris Masters to have a shot at qualifying for the ATP World Tour Finals.
Del Potro increased his chances of making the season-ending event in London by beating American Sam Querrey 6-2, 7-5. The five players ahead of Del Potro dueling for the final three places have either lost or withdrawn from their matches this week.
Del Potro dominated the first set and broke Querrey late in the second before winning the last game with his seventh ace of the match.
"I was a little nervous in the second set, but I ended up hitting some great forehands," del Potro said. "(Granollers) is going to be tough, he's coming in with a lot of confidence."