"I was running to the left and I felt some stretched muscles and (it was) painful," said Davydenko, who had received three minutes of medical attention at the close of the first set. "Maybe I can finish match but I don't want to lose 6-2. I have the Masters tournament coming up, so what can you do? It's really tough."
Federer had set up five break chances over Davydenko's first three service games but hit unforced errors on four of those chances, while Davydenko, who preferred to sit back along the baseline, fired one of his three aces to save the other.
Federer's serve was never under question coming into Estoril and he served an ace past Davydenko to save a break chance in the eighth, with Davydenko shooting a low forehand into the net for Federer to hold it at 4-4.
Federer, who was springing to the net continuously, was down 0-30 to start the 10th but recovered only to face set point which led to his fourth ace. Federer took it to a tiebreaker after saving another set point when Davydenko shot out.
After trailing 3-0 in the tiebreaker, Davydenko rallied with a forehand winner down the near line to level at 5-5. After hitting a crosscourt forehand for his 12th winner of the opening frame, Federer clinched it after Davydenko hit a forehand into the netting.
Davydenko had come in on the heels of a title victory over No. 2 Rafael Nadal at Miami.
Maria Kirilenko beat Iveta Benesova 6-4, 6-2 to win the women's crown -- her third career title.
The second-seeded Russian broke Benesova's serve five times, helped by the Czech's five double faults. It was the third straight tournament that Benesova lost in the final.
Neither woman held serve confidently in the first set with two breaks apiece going into the 10th game with Kirilenko leading 5-4.
But Kirilenko picked her spot for a forehand winner along the line for 30-30. Benesova then shot wide to set up a first set point before rain forced an interruption.
After the 41-minute delay, Benesova, who sits 100 spots below Kirilenko in the rankings, netted from a long rally to hand the 32nd-ranked Russian the first set.
Kirilenko exploited Benesova's lack of rhythm to serve and break to love for 2-0 in the second set. Kirilenko had won 10 straight points before the 25-year-old Benesova smacked a forehand inside the line to start the third game.
Kirilenko won with her last break in the eighth game when Benesova returned into the net.
Kirilenko, 21, had easily won their only previous meeting, also on clay, at Warsaw in 2005.
(Agencies April 21, 2008)