Shannon Boxx's late goal on a header was the difference in the game, the first meeting between the teams since Sundhage left the China national team, where she was an assistant coach under countrywoman Marika Domanski Lyfors at the 2007 Women's World Cup.
After its 4-1 win over Finland on Friday, the Americans only needed a tie on Sunday to win the tournament, but the Chinese held back, forcing Sundhage to make a switch at halftime.
Tobin Heath came for Lauren Cheney and moved to the left wing, while Heather O'Reilly moved up to play right wing in a 4-3-3 formation with Heath and Abby Wambach on the frontline.
Lori Chalupny, a substitute for O'Reilly in the 72nd minute, created the free kick that would decide the game. The initial service was cleared off, but the ball fell to defender Becky Sauerbrunn on the right wing and she sent the ball back into the middle, where Boxx scored on a looping header from 11 yards out for the 18th goal of her international career.
Sauerbrunn had returned to the starting lineup after breaking her nose in a collision with a Canadian player in the opening game.
Heath, Sauerbrunn, Ali Krieger, who started the first two games, and Tina DiMartino all made their international debuts at the Four Nations Tournament. Cheney and Angie Woznuk, who also came off the bench against China, came into the tournament with two caps and one cap, respectively.
Sundhage was pleased with the USA's play after halftime.
"The hardest part is to penetrate," she said, "and that's why we changed in the second half and put three up top, played 4-3-3 and tried to get as wide as possible. A player like Tobin Heath on the left side did great, and Heather O'Reilly on the right side also had a good performance. It was nice to see our speed and eventually we got that goal."
Sundhage was pleased with the play of her young players and those coming off the bench.
"It's great," she said, "because I am telling them over and over again that the bench is important. It's not about 11 players or one coach, it's about the team. So the fact that they are coming off the bench and doing a great job to make the difference, that is so important. The games we will go and play, we will need everybody."
Jan. 18 in Guangzhou
USA 4 Finland 1. Goals: Tarpley 37, 39, Cheney 69, Woznuk 80; Sainio 54.
USA -- Scurry, Cox, Rampone, Krieger, Tobin, Boxx (Woznuk, 46), Lloyd (Osborne, 74), Tarpley, O'Reilly (Chalupny, 72), Rodriguez (DiMartino, 63), Wambach (Cheney, 46).
Finland -- Korpela, Hyyrynen, Julin (Westerlund, 76), Valkonen, Vaelma, Salmen, Sainio, Puranen (Nokso-Koivisto, 76), Rantanen (Sjolund, 83), Lindstrom, Tolonen (Sallstrom, 46).
Referee: Gu Ying (China).
Jan. 20 in Guangzhou
China 0 USA 1. Goal: Boxx 77.
China -- Zhang Yanru, Yuan, Li, Wang, Zhang Ying, Zhang Tong, Bi, Zhang Na, Xie (Zhang Cheng, 80), Liu (Guo, 69), Han.
USA -- Solo, Cox, Rampone, Sauerbrunn, Chalupny, Boxx, Lloyd (Hucles, 66), Tarpley (Woznuk, 46), O'Reilly, Cheney (Heath, 46), Wambach (Rodriguez, 76).
Referee: Carol Anne Chenard (Canada).