The 37-year-old center took control in overtime for a second consecutive game, this time scoring the first five points as the Spurs rallied from an 18-point deficit to beat the Memphis Grizzlies 104-93 Saturday night.
That put them a win away from their first finals since 2007.
"He was huge for us the other night in overtime and got it done for us, and he did it again tonight," Spurs coach Gregg Popovich said of the two-time MVP. "That's why he is considered as great as he has for the last 17 years. He's been unbelievable. He feels a responsibility to carry us in those kinds of times, and he did it again tonight."
Duncan wants to get back to the finals after being on the verge last year. He said he was more aggressive in overtime after being a bit tentative with his shot in regulation.
"I'm not worried about how old I am or whatever, whatever it may be," Duncan said. "I'm very focused on having another opportunity to make it to a championship and try to win, that's all."
The Spurs grabbed a 3-0 lead in the Western Conference finals and now have won five in at row this postseason. With the memory of blowing a 2-0 lead a year ago in the West finals to Oklahoma City, when they lost the next four, the Spurs shook off their sloppy play early and pushed the young Grizzlies to the edge of elimination in the first West final played in Memphis.
The Grizzlies knocked off the Clippers after trailing 0-2 and upset the West's top seed in Oklahoma City in the semifinals. Now their task is even tougher with Game 4 in Memphis on Monday night.
"We've been fortunate to get this far, and we've got to play better if we want to continue," Grizzlies coach Lionel Hollins said.
Memphis set up for the final shot to win in regulation, but guard Mike Conley missed on a runner at the buzzer. He said he went a little too deep in the paint on the shot.
No NBA team has lost a series when leading 3-0.
"It is frustrating, but you know it gives us hope at the same time knowing that we can fight our way back," Conley said. "And we have had chances to win each of these games. This was a tough one to swallow, but it was a good learning lesson. There's no group of guys I'd rather be with down 3-0 to try to fight back in it."
San Antonio, which didn't lead this game until the opening minute of the fourth quarter, can wrap up the series Monday in Memphis and get back to the finals for a shot at their fifth title as a franchise.
"One of the best wins I've witnessed being a Spur," Spurs guard Manu Ginobili said.
The Spurs hit eight of their 10 shots in overtime, with Duncan scoring seven of his 24 points. He also had 10 rebounds. Tony Parker had five of his 26 in overtime, and even Tiago Splitter, playing with four fouls, scored six in the extra five minutes to finish with 11.
"For whatever reason, we didn't panic," Parker said of a start that included four turnovers in the first quarter. "We knew they wanted to come with a lot of energy in the first half. We played our game and came back."
Mike Conley led Memphis with 20 points. Marc Gasol had 16 points and 14 rebounds, Zach Randolph added 14 and 15, and Quincy Pondexter had 15 points. But the Grizzlies, who thrived at the free throw line in knocking off Oklahoma City, got there only 18 times and made only 10.
The Grizzlies last led 85-84 with 1:04 left in regulation on a 15-footer by Gasol. After that, they managed only to tie it up twice, the last on a layup by Randolph with 4:28 left in overtime. Duncan scored and knocked down the free throw with 4:10 remaining to put the Spurs ahead to stay.
The Spurs dominated the Grizzlies in the paint, outscoring Memphis 58-42 to offset their 17 turnovers, which the Grizzlies turned into 25 points. After the Grizzlies outscored San Antonio in the first quarter, the Spurs outscored them in each of the final three periods and overtime, where they had an 18-7 edge to put away the win.