Alonso fielded a blend of first-team experience and academy youth: Teddy Sharman-Lowe started in goal behind a back four of Tayo Subuloye, Josh Acheampong, Tosin Adarabioyo and Landon Emenalo. Sixteen-year-old Reggie Watson lined up alongside Essugo in midfield, Reggie Walsh took the number 10 role, and Ryan Kavuma-McQueen, Dastan Satpayev and Liam Delap formed the front line.
Chelsea broke the deadlock inside five minutes: a passing move through Sharman-Lowe, Acheampong and Subuloye released Kavuma-McQueen, who spun away from his marker and slid a pass through for Satpayev to race onto and finish. Western Sydney levelled through Anthony Pantazopoulos, converting from close range after a corner was flicked on, moments after Sharman-Lowe had parried an earlier effort from Liam Gillion.
The hosts then edged ahead as Hammond skipped past Acheampong and beat Sharman-Lowe at the near post, only for Essugo to restore parity within six minutes, sweeping home after a one-two with Walsh on the edge of the box. Adarabioyo also went close before the break, heading a Watson corner narrowly wide.
Western Sydney regained the lead just after the hour when Emenalo's clearance rebounded kindly for Dylan Scicluna to finish. Alonso responded by sending on nine substitutes at once, and the fresh legs transformed the game.
Gittens levelled almost immediately, finishing off a driving run from Joao Pedro, before Awan Lual restored the hosts' lead. Joao Pedro drew Chelsea level again straight away, then completed his hat-trick with a composed finish from 18 yards and a close-range header from an Estêvão Willian corner to seal the win.
Reflecting afterwards, Alonso said the game had offered valuable lessons ahead of the new season.
"I think that this friendly is going to be useful for the future," he said. "There were moments in the game that we were struggling to find solutions, and we need to have this experience.
"The positive is that we corrected them, but these moments of game management are something we can learn from today. When they were pressing so high, it was tough to find a solution from inside.
"We have to have a different understanding of what's happening and try to make those changes in the game, as it's something that will happen often. The more situations we have where we are struggling and we find the solutions, the better team we will be."
The Spaniard also pointed to the wider benefits of the night.
"So, no injuries, the young players having a good experience of coming on the tour and playing minutes, the senior players came on for good minutes and made a good impact, and it was an exciting game for the fans as well."
Romeo Lavia was the only member of the matchday squad not to feature, with Alonso confirming it was precautionary: "He felt something in the hamstring. Not a big thing and, hopefully, we are expecting him to be fit for Saturday. But there was no point in taking risks today, which is why Reggie Watson played 90 minutes and did well."
Line-ups:
Chelsea: Sharman-Lowe (Sanchez 63), Subuloye (Palestra 63), Acheampong (Fofana 63), Tosin Adarabioyo (c) (Colwill 63), Emenalo (Anselmino 63); Watson, Essugo (Nicoll-Jazuli 46); Kavuma-McQueen (Kellyman 46, Estêvão Willian 63), Walsh (Palmer 63), Satpayev (Gittens 63); Delap (Joao Pedro 63). Subs not used: Curd, Diakite, Lavia.
Western Sydney Wanderers: Holmes (Vidackovic 46), Gersbach (Morrison 70), Pantazopoulos (Fransen 64), Ferreyra (Barrie 45), Madanha (Katrib 64), Scicluna (Ugarkovic 64), Thurgate (Rose 68), Di Pizio (Carluccio 64), Gillon (Waraga 46), Borrello (Lual 64), Hammond (Milanovic 46). Subs not used: Thomas, Abdul-Rahman, Cetinic, Hassarati, Savic.