SAN FRANCISCO -- Nationals reliever Luis Ayala hasn't been effective all season, and his problems continued on Wednesday night. He gave up three runs in the bottom of the eighth inning as the Giants defeated the Nationals, 6-4, at AT&T Park.

Ayala has been healthy, but, for some reason, he has been reluctant to throw his best pitch -- the sinker -- which was clocked at 94 mph during the game. Instead, Ayala got beat on his fourth-best pitch, the changeup, in the eighth inning.

It was bad the moment Ayala hit the mound. The first batter he faced was catcher Bengie Molina, who was behind in the count, 0-2. On the next pitch, Molina singled.

After getting Aaron Rowand to fly out to Willie Harris in center field, Ayala walked John Bowker to put runners on first and second. Rich Aurilia stepped to the plate and took a 3-2 change up and doubled to left-center field to score pinch-runner Emmanuel Burris and Bowker to give the Giants a 5-4 lead.

"They beat him with his fourth-best pitch, so he's struggling. We played a good game up until that point," manager Manny Acta said. "[Using his fourth-best pitch] was his choice. He battled Rich with his sinker and gave in with that changeup."

Aurilia would then come home on a double by Omar Vizquel. The poor outing raise Ayala's ERA to 5.77 for the season, and manager Acta hinted that Ayala would no longer be one of the setup guys in the late innings.

"He hasn't been successful there," Acta said. "Now, losing [closer] Jon Rauch and moving everybody up, we are going to go with who has the hot hand and matchups."

In order for Ayala to be successful, he must fix his mechanics as well. The front side of his body is flying open.

"I try to stay more closed, so it's hard now, but I'm going to keep working," Ayala said. "It's easy to make a mistake. That is going to happen."

There also is the confidence issue. All season, he has been reluctant to throw the sinker. Pitching coach Randy St. Claire said Ayala now throws the pitch about 50 percent of the time.

In the past, there was a good chance a hitter saw the sinker 80 percent of the time. One source believes that Ayala has problems dealing with the fact that he will become a free agent after the season. Whatever the problem is, Ayala needs to fix it fast.

"He has been getting hurt on a lot of offspeed stuff," St. Claire said. "He has to be confident in his No. 1, which is his sinker. That is his best pitch. It always has been. It probably always will be.

"He is working behind hitters; he is walking guys. You can't do it in the eighth and ninth innings. ... You just can't do it. They score. ... I think he has to be more aggressive with the strike zone and use his [sinker]. That's the type of pitcher that he is. His arm strength is there. He threw a good sinker by Rich Aurilia that he had no chance on -- 94 sinker and then throws a hanging something over the middle of the plate and he lines it into the gap."

The game looked promising heading into the bottom of the eighth. Nationals right-hander Collin Balester got off to a slow start, as the Giants scored two runs in the first inning to take a 2-0 lead. After the fifth inning, Balester left the game behind, 3-1. But the Nationals came back and took the lead in the top of the sixth off right-hander Kevin Correia.

With Austin Kearns on first base, Jesus Flores hit a two-run homer over the left-field wall to tie the score at 3. Four batters later, Johnny Estrada singled to center to drive in Ryan Langerhans with the lead Ayala couldn't hold.