The Dane edged a gruelling first set which lasted almost an hour, but then raced through the second to end the challenge of 10th-seeded Petkovic 7-5 6-1.
Wozniacki, seeded two in Eastbourne, will come up against Belinda Bencic in the last four after the Swiss came from behind to knock out home hope Johanna Konta.
The British wild card had shown amazing form to reach the quarter-finals but found world number 31 Bencic a bridge too far, with the 18 year old emerging a 2-6 6-0 6-3 winner.
Konta told the WTA Tour website: “Obviously I played a very solid, good first set, and started the match really well.
“She definitely stepped up her level after that. She played very well. All credit to her – she played tough and solid when she needed to, and she just played the situation – she was definitely the better player today.
“But I’m really happy with my effort this week. I’m really happy with the week in general. Today I just played against a player who was very tough – she stayed tough, and she came out on top in the end.”
The other semi-final will pit ninth seed Agnieszka Radwanska against Sloane Stephens of the United States.
Poland’s Radwanska enjoyed a straightforward 6-2 6-2 victory over Bulgarian Tsvetana Pironkova while Stephens advanced via a walkover after her opponent, Russian Daria Gavrilova, pulled out with an abdominal injury.