Tennys closed the first game of the fourth set with a booming serve, still standing strong in the service games after that break he suffered in the opening set. Federer stayed in touch early on with a service winner in game two, desperate for some chances on the return despite all the problems.
That had to wait for some other game, though, as Sandgren made another comfortable hold with a forehand winner in game three, sending the pressure to the other side of the net and gaining more and more energy with every successful service game.
Battling for another game, Federer held at 30 with an unreturned serve to level the score at 2-2, giving his best to stay in contention as long as possible. Still, there was nothing left for him in the return games, with another quick hold at love for Sandgren who moved 3-2 in front, hoping to seal the deal with one successful performance on Roger's serve.
Two service winners got Federer out of jail in the sixth game, leveling the score at 3-3 and passing the first half of the set without damage we had seen in sets two and three. Confident in his shots, Sandgren delivered another hold at love in the seventh game with a forehand winner, sending the pressure back to Federer and doing everything right so far against the great champion.
Roger survived two deuces in the eighth game to level the score once again before Sandgren landed another ace to grab the next game and move 5-4 in front, forcing the Swiss to serve for staying in the match. Federer opened a 40-15 lead before losing the next three points, fending off a match point after a backhand error from Tennys who couldn't convert the second chance as well following a forehand mistake.
The American netted a forehand on the third match point, with Federer making a crucial hold with a forehand winner to hang in there. With the momentum on his side, Rofer earned a break point in the 11th game with a forehand down the line winner, unable to convert it as Sandgren held for a 6-5 lead after a bold second serve.
Federer held at 15 to bring on the tie break where they both served well in the first six points before Roger netted a forehand to send the American 4-3 in front. Two service winners sent Tennys 6-3 in front before Federer erased match points four, five and six for more drama and excitement!
Serving at 6-7, Roger denied the seventh match point and earned a set point with a smash winner that Tennys erased with an ace in the best possible moment! The Swiss endured a long rally to grab a mini-break and move 9-8 in front, closing the set for a massive boost ahead of the decider.