Duncan, who turned 40 in April, announced Monday that he is retiring from the NBA after 19 seasons, all with San Antonio.
"More cutthroat than people give him credit for," Los Angeles Lakers legend Kobe Bryant told ESPN's Marc Stein on Monday upon learning of Duncan's retirement. "I loved everything about him on the court."
Duncan's final game ended up being a 113-99 loss to the Oklahoma City Thunder in Game 6 of the Western Conference semifinals on May 12. Quiet for much of the series, Duncan showed flashes of his All-Star form in what turned out to be his career finale, with 19 points, five rebounds and a block in 34 minutes.
With the Spurs getting blown out in Game 6 and the fourth quarter set to begin, coach Gregg Popovich and his veteran star had a brief conversation on the bench. Duncan then played all 12 minutes of the fourth quarter without coming out for a rest, perhaps soaking up every second he could in the final game he would ever play. When the game was over, Duncan waved to the visiting crowd and pointed a finger toward the roof as he headed to the locker room, a rare signal from one of the league's most stoic superstars.
"Timmy's never been a very outspoken or emoting sort of individual on the court," Popovich said earlier this year. "Everybody does it differently."
Duncan partnered with Popovich to post the most wins by a player and coach in NBA history with 1,001. The Spurs coach will discuss Duncan's decision to retire at a news conference Tuesday. There are no plans for Duncan to address the media.
Commissioner Adam Silver issued a statement about Duncan's announcement, saying, "Tim Duncan is one of the most dominant players in NBA history. His devotion to excellence and mastery of the game led to five NBA championships, two regular-season MVP awards and a place among the all-time greats, while his understated selflessness made him the ultimate teammate.
"For two decades Tim represented the Spurs, the city of San Antonio and the league with passion and class. All of us in the NBA family thank him for his profound impact on the game."
Since drafting Duncan, the Spurs posted a 1,072-438 regular-season record. That is the best 19-year stretch in NBA history and tops in the four major U.S. sports over the past 19 years.
"Man, he's meant a lot, a great amount," said Spurs small forward Kyle Anderson, one of Duncan's 140 teammates over the years. "When you have Timmy on the floor and you're out there, it's so easy to give all your effort because you know he's just out there talking, he's out there making sure everybody's playing hard.
"He's like, I don't want to say a father figure out there, but he's like a big brother out there. I love Timmy. He's been a great teammate."
No group won more regular-season or playoff games than those three. According to the Elias Sports Bureau, they were the first set of three or more teammates to win four titles together since Magic Johnson, Kareem Abdul-Jabbar, Michael Cooper and Kurt Rambis.
Ginobili tweeted that it was an honor to play with Duncan.
Former rivals were effusive in their praise of Duncan upon learning of his decision to step away from the game.
"Greatest power forward of all time," Shaquille O'Neal told Stein. "Unbreakable power forward. No [elbow] could break him. No loss of a championship could break him. Nothing could break him."
LeBron James, who lost to the Spurs in his first trip to the Finals, took to Twitter to praise Duncan.
Golden State Warriors coach Steve Kerr, who played with Duncan for four seasons, told Stein it will be strange not to see Duncan on the court.
"When you think of a Spurs game, you think of the opening tip and Timmy cradling the ball and looking down at Pop and Manu and Tony," Kerr said. "The four of them really kind of define who they are. But Tim is the main guy obviously.
"They'll still be the Spurs based on what they've built. And maybe that's Timmy's lasting legacy. He helped build something so strong that's still going after he leaves."
A four-year star at Wake Forest, Duncan was the No. 1 overall pick by the Spurs in the 1997 draft and made an immediate impact, winning the NBA Rookie of the Year award and earning All-NBA First-Team honors, the first of 10 selections in his career. He led the Spurs to a championship in his second season, in 1999, and was named Finals MVP.
He would go on to win four more titles, in 2003, 2005, 2007 and 2014, and was named Finals MVP twice more, in 2003 and 2005. San Antonio posted a win percentage of at least .600 in 19 straight seasons with Duncan, the longest such run in NBA history.
Duncan finishes his career with averages of 19.0 points, 10.8 rebounds, 3.0 assists and 2.2 blocks per game. He ranks 14th all time in points (26,496), sixth in rebounds (15,091) and fifth in blocks (3,020).
Duncan made the NBA All-Defensive First Team eight times. He is only the third player in NBA history to win 1,000 career regular-season games. Abdul-Jabbar and Robert Parish are the only players with more career victories.
With the retirements of Duncan and Bryant, Minnesota Timberwolves forward Kevin Garnett is the final player in the NBA who played in 1997-98. Garnett, 40, joined the league in 1995 and is signed through next season.
ESPN staff writer Michael C. Wright and The Associated Press contributed to this report.