Tello's late effort did justice to the score as Betis had twice hit the woodwork earlier in the game through Guido Rodriguez and Sergio Canales.
Osaka then gained the momentum and broke a third time to take the match to a deciding set.
Club co-owner Jorge Mas posted a picture of himself with the 32-year-old at Miami international airport and an official announcement confirming his move was said to be imminent.
The captain of the Chinese women's volleyball team registered this week at Beijing Normal University, pursuing a master's degree in history.
"It's all or nothing," Thiem said. "It's going to be a completely open match. I'm really looking forward to that. We have such a great friendship and rivalry. It's amazing that we're going to face each other in a Grand Slam final."
SUZHOU, China, Sept. 11 (Xinhua) -- Matheus Nascimento's penalty at his 100th game in Shijiazhuang Ever Bright handed his team a 1-0 win over Wuhan Zall in the 10th round of the Chinese Super League (CSL) here on Friday.
The region's premier club tournament, the Copa Libertadores, was due to resume on Tuesday, having been suspended in mid-March.
A concertina effect after the first start saw the slow-starting Red Bull of Max Verstappen - who had had engine trouble before the race had even started - and Pierre Gasly's AlphaTauri pushed off into the gravel at turn 2, resulting in the deployment of the safety car to clear the stricken vehicles.
In other fixtures on Thursday, Palmeiras won 2-0 at Corinthians and Atletico Goianiense beat Vasco da Gama 2-1 in Rio de Janeiro. Enditem
Former Atletico Madrid and Chelsea defender Filipe Luis put Flamengo ahead with an eighth-minute strike and Barbosa doubled the lead with a powerful finish after Muriel Becker parried away a free-kick.
Luneng were dealt a blow in the 20th minute as Marouane Fellaini picked up an injury, making way for Roger Guedes.