had been below his best all week in Portugal
Roger Federer won his first title of the year after his opponent in the Estoril Open final, Nikolay Davydenko, was forced to retire with a leg injury.
The world number one survived a set point in the first set before winning it, and was a break down in the second.
When Davydenko pulled out after 79 minutes, Federer led 7-6 (7-5) 1-2.
Earlier, Russia's Maria Kirilenko enhanced her clay-court credentials by winning the women's final, beating Czech opponent Iveta Benesova 6-4 6-2.
Federer, who suffered glandular fever before the Australian Open, was tested by Davydenko but the Russian's injury helped him make it 12 wins and no losses against the world number four.
Davydenko said afterwards: "When I ran to the left, I felt pain. I tried to fight but I knew there was not much that I could do."
Federer commented: "It's not the way you want to win a tournament but that's just the way it goes sometimes.
"It's great to win a title again and to straight away win my first clay-court tournament of the season gives me great confidence going into Monte Carlo.
Kirilenko needed the trainer to strap her right thigh
"These were the most difficult [conditions] of the week.
"It was cold as well as windy. This is not the best way to win a match and I'm sorry for Nikolay. But sometimes these things happen."
After a difficult hardcourt campaign, Federer had employed the services of Spanish coach Jose Higueras for the clay-court season.
He dropped sets against low-ranked players in two previous rounds during the week, and after failing to convert five break-points in the first set of the final, had to dig deep to avoid losing it.
Davydenko, who began the day on a run of 11 straight wins, was building some momentum when breaking Federer early in the second set, before his untimely withdrawal.
Federer now travels to the Monte Carlo Masters as he prepares for his bid to win the French Open in May, the only Grand Slam title that has eluded him.
Davydenko said he expected to recover in time to play in Monte Carlo as well, with the event starting on Monday.
The second-seeded Kirilenko, 21, broke Benesova, the world number 132, five times to secure her third career title.
The Muscovite, ranked 32 in the world, needed to have her thigh strapped at one point but her game proved too strong for Benesova to mount a challenge.