West Indies captain Brian Lara was a relieved man after his side beat South Africa by seven wickets in the fourth one-day international on Sunday to keep the five-match series alive.

West Indies had lost the first two encounters after batting second under floodlights, but Sunday's game at Centurion was the first in the series not played as a day-nighter.

"Finally, we got a game where the toss didn't matter," Lara told reporters after the tourists had overhauled the victory target with five overs to spare.

"It's a bit disappointing that we will play four of the five games under lights. It's unfair to both teams."

South Africa lead the series 2-1 going into the final floodlit match in Johannesburg on Wednesday, when Lara believes the side winning the toss will hold an unfair advantage.

"Conditions are so much in favour of the side bowling second, it's ridiculous," he said, after smashing an unbeaten 59 off just 37 deliveries in a West Indies total of 300 for three.

"We're going to the Wanderers on Wednesday, and you wonder what's going to happen at night. I'm almost sure the toss is going to be a factor."

Lara scoffed at South Africa captain Graeme Smith's claim that the home side were "mentally fatigued" after a four-match Test series against West Indies followed by four one-day internationals.

"Graeme Smith loses one game against us and his team is tired?" Lara said. "West Indies are fighting on, we've had so many injuries and haven't complained."

South Africa piled on 297 for four in their 50 overs after winning the toss on Sunday but Smith said his team had under-performed with the ball.

"We made a decent enough score, but on very good wickets you have to bowl with precision and we weren't precise enough," he said. "We bowled both sides of the wicket."

Smith was, however, fulsome in his praise for the West Indies batsmen. Along with Lara's 59 not out, opener Shivnarine Chanderpaul struck a whirlwind 92 and Ramnaresh Sarwan hit an unbeaten 77.

"Chanderpaul hit into a lot of weird, different areas, and when you've got a man like Brian Lara walking to the wicket with a run a ball needed, you're going to struggle if you're the bowling side," he said.