Pavel has not won a title since his victory at the Tennis Masters Series Canada in Montreal last July. He won for only the second time in five meetings with Portas.
The 28-year-old from Constanza, Romania will be looking to reach his second semifinal of the season Friday when he takes on Spanish qualifier Juan Giner. Pavel was a semifinalist at Chennai in January and a quarterfinalist at four other events, including the French Open.
"Finally I'm breaking the ice," Pavel said. "I reached the semifinals once (in 1996) and the quarterfinals in 1998, otherwise I have always lost in the second round. It feels great to be in the quarterfinals here and to have won against two of the best players in the draw, (Alberto) Martin and Portas.
"I feel good and I'm confident."
Giner gained revenge against Irakli Labadze of Georgia with a 6-3, 6-2 victory on Thursday. Labadze rallied for a three-set win over Giner in the second round here last year.
"I don't know anything about (Giner)," Pavel said. "He's a new guy. But he must have played well coming through qualifying and winning two matches."
Argentina's Franco Squillari ousted fourth-seeded Fernando Vicente of Spain, 7-6 (8-6), 5-7, 6-1. Squillari defeated Vicente for the first time in five encounters.
"I played unbelievable today," Squillari said. "This win gives me a lot of confidence, it's my first win against him. I was the stronger physically at the end of the match. That was the key to the victory."
Fifth seed Jose Acasuso inched closer to his second title of the year with a 6-4, 6-2 triumph over Romanian wild card Victor Hanescu. Acasuso won at Sopot in July.