They watched the rookie fly out to center field in his first Major League at-bat in the second inning, then later drive Yankees left-hander Andy Pettitte's pitch into right field for a double in the fourth inning -- his first career hit. He later scored his first big league run on Miguel Cairo's groundout.
"The first day in Texas [on Tuesday] was a big day for him, because he was in the big leagues as of that day," said Mariners manager Jim Riggleman, "but [Friday] is another big thrill for him."
The biggest thrill, so far.
The two games that Tuiasosopo watched the Mariners play in Arlington count as far as time of service, but the series opener against the playoff-contending Yankees beats anything the third baseman has experienced since being drafted in the third round of the 2004 First-Year Player Draft.
"I never put a timeline on myself," he said, "but it definitely feels good to be up here."
Riggleman told reporters in Arlington on Wednesday that he was thinking about starting Tuiasosopo against Pettitte in Friday night's series opener. But he didn't say anything to the rookie, who had a breakthrough August at Triple-A Tacoma, batting .337 with five doubles, seven home runs and 19 RBIs. Overall, he batted .281 with 32 doubles, 13 home runs and 73 RBIs.
"I was pretty sure that I was going to play him, but I wasn't positive," Riggleman said prior to Friday night's game. "I didn't want to tell him he was playing, and then he gets here and he doesn't play. But I probably wouldn't have told him anyway and have him thinking about it for a couple of days, with the off-day and all."
But many of Tuiasosopo's friends heard about the potential start and sent text messages, asking if he would, indeed, be in the Mariners' starting lineup.
"I was hoping [to play] in Texas," he said. "I didn't, and I'm definitely excited to be in there tonight. I had heard that I might be [starting], so I was at home trying to find some video [on Pettitte]. I finally found some of him on YouTube and he was nasty. I think he struck out everyone.
"I didn't know for sure until the lineup was posted."
He was so excited that he wasn't able to sit or stand still after looking at the bulletin board with the lineup with his name in the No. 7 spot between Wladimir Balentien and Kenji Johjima.
"Hopefully, he has a good day, and it will be something he can reflect on and be happy about," Riggleman said. "I've had the pleasure of doing this with a few guys, and it's a big thrill."
Exactly how many more starts Tuiasosopo gets during the final 22 games of the regular season remains uncertain. Riggleman put regular third baseman Adrian Beltre in the designated-hitter spot on Friday night, and the Gold Glove winner figures to get most of the playing time at third for the next three weeks.
"I don't know yet how much he will play," Riggleman said. "Just like the other [callups], we'll get them in there when we can, but I'm not going to ignore the fact that our veterans have played hard for us all year, and I won't take September away from them."
There promises to be many Septembers in the future for Tuiasosopo, who was drafted as a shortstop but moved to third base early in his professional career. He has the physical stature (6-foot-2, 223 pounds) to develop into a middle-of-the-lineup run producer.
The most successful corner infielders are those that can drive balls into the gaps and hit home runs. Riggleman envisions Tuiasosopo becoming one of those players.