Nadal was off form against the Russian in Qatar
Nikolay Davydenko beat an ill Rafael Nadal 6-3 6-2 to set up a Qatar Open final date with Roger Federer.
The Spanish world number one, who lost eight straight games in the match, said afterwards that he was suffering from the effects of a week-long fever.
Earlier, Federer defeated the third seed Jo-Wilfried Tsonga 6-3 7-6 (7-2).
Two-time champion Federer broke Tsonga in the second game of the first set and was 2-2 in the second-set tie-break before taking the next five points.
In the second semi-final, Nadal, who has been on antibiotics for several days, was level at 3-3 in the first set before losing eight successive games to Davydenko.
I felt real bad, but, you know, is difficult for me to go off court
Nadal finally won a game on his serve for 5-1 before breaking Davydenko but in the next game the Russian booked his passage to the final by taking his third match point.
"I thought about retiring for a moment. I had no chance to win the match," Nadal said. "I knew that after four games, five games.
"I felt real bad, but, you know, is difficult for me to go off court. It's not nice for the crowd, not nice for the tournament, not nice for the opponent, too, no?"
Nadal will stay in Doha for two more days to recuperate before the start of the Australian Open on 17 January.
If he wins the tournament in Melbourne he will become the first man since Rod Laver in 1969 to hold all four Grand Slam titles at the same time.
"I stay here because I have to get healthy before I travel," he said.
"Going by plane sometimes makes you worse. And at the same time I don't have anything to do right now in Australia."
Davydenko, who beat Nadal in the final last year, was surprised by his lack of stamina.
"When I played him last year he had good form," Davydenko said. "Today he felt different. Normally he is tougher and he plays stronger. I don't mean his tennis, I mean physically."
Meanwhile, Federer said he was looking forward to the showdown against Davydenko.
"Obviously I'm very excited. It's always nice to start off the year with a final," said the Swiss player, who won in 2005 and 2006.
"I think Nikolay actually has a very good record in finals, and that makes him tough to play in a final. I feel well and I'll definitely have a chance with all my experience and the level I'm playing at right now."
The 29-year-old said the early advantage in the first set helped set the tone of the encounter.
"It was a crucial service break for me. That set me up nicely for the remainder of the match," he added. "The second set was more competitive as I was serving really well."