The host's efforts paid off with 17 minutes played as Mainz' Leandro Barreiro fouled onrushing Dani Olmo inside the area. Forsberg broke the deadlock with a well-placed shot from the spot into the bottom right corner.
Andre Villas-Boas' side beat PSG 1-0 last Sunday, ending a nine-year winless run against the old rivals, but suffered a bitter 2-0 defeat to St-Etienne on Thursday.
Opponent Real Betis has started the campaign well under Manuel Pellegrini and the former Real Madrid coach looks to have added steel to a side which flattered to deceive last season.
Suzhou pulled one back in the 60th minute with an accurate cross from midfielder Li Haowen, assisting forward Tang Chuang to score an open goal.
Other matches on Wednesday saw Shandong smashed Sichuan 3-0 (25-13, 25-17, 25-17), Fujian beat Beijing 3-1 (28-26, 28-26, 19-25, 25-10), Zhejiang edged Guangdong 3-2 (25-20, 25-13, 22-25, 22-25, 18-16), and Liaoning overcame Henan 3-2 (20-25, 24-26, 25-10, 25-11, 15-6). Enditem
The top four finishers of each group will form a new group, to be based in Suzhou, to fight for the first to eighth position of the season in the second phase, which will be held between October 16-December 12.
Earlier this month, Costa Rican team officials said the match would not be possible if the country's health ministry did not provide an exception to a mandatory 14-day quarantine for all travellers arriving from abroad.
Fans and half of the club are puzzling about who could be the rescuer leading the side out of this deep crisis. Former Schalke and Leipzig coach Ralf Rangnick is one name in the list.
With their largest-ever win in the AFC Champions League, Sharjah could reach the Round of 16 for the first time in their history if they win against Persepolis in their last game, while Al Taawoun's fate is now out of their hands as they need a win against Al Duhail and a draw between Sharjah and Persepolis to qualify.
The Celta captain opened the scoring after receiving a pass from Nolito after 15 minutes in an entertaining match which saw Celta play some impressive football.
Although both Li and Wang were substituted in the third set, Tianjin's young players Wang Yizhu and Chen Boya combined for 10 points to sail to the victory.