The world No. 4, and two-time finalist at Roland Garros in 2018 and 2019, reached the fourth round in Paris for the five consecutive years.
Houssem Aouar converted a penalty in the 28th minute to equalize for the hosts after Melvin Bard was taken down by Alvaro Gonzalez.
Mainz failed to put up any resistance while Union relentlessly tripled their lead after Marvin Friedrich headed home Christopher Trimmel's free-kick in the 63rd minute for 3-0.
"If every player can contribute, it will be an easier game for us," she noted.
In the men's 1,500m race, world leader Cheruiyot fought off compatriot Kumari Taki in a fierce home straight brawl to take the win in three minutes and 34.31 seconds in front of 2000 spectators that were allowed the entry under COVID-19 control protocols.
NANNING, Oct. 4 (Xinhua) -- Swimming enthusiats in Liuzhou, a river-rich city of south China's Guangxi Zhuang Autonomous Region, attended their first river-crossing competition since the COVID-19 outbreak.
Shanghai managed to turn a six-point deficit after the first quarter into a 10-point lead at the end of the first half, and never looked back afterwards.
The two-time Grand Slam winner committed just seven unforced errors throughout the one-sided encounter and broke her opponents six times.
American Sara Hall, who sprinted to surpass world champion Ruth Chepngetich in the last meters, finished second in 2:22:01. Chepngetich from Kenya completed the race four seconds later.
"We played a bit tight at the start as it was our first game, but we never gave up, and finally won the game," Li said.
"That's our aim and I'm confident we can compete with the footballers we have," he added, despite the refusal of several clubs to release their players for national duties.