GUY kicks off the start of Wimbledon…
A very entertaining set of tennis themed clues. I'm slightly unsure about the parsing of 27a.
Thanks GUY!

ACROSS
1. SW19, a sign summer’s here (7)
SWALLOW
SW + ALLOW (let, 19 across)
5. 25 cheer as C Alcaraz holds (6)
RASCAL
[chee]R AS C AL[caraz] (holds)
25 (down) being KNAVE
8. Flexible Serb with limitless stamina showing nerves (9)
BRAINSTEM
(SERB with [s]TAMIN[a] (limitless))* (*flexible)
9. Frame of amateur racket that’s notched for string (5)
ARROW
A[mateu]R (frame of) + ROW (racket)
11. More than one round, sounds clear (5)
RINGS
Double definition
A ring being a round
12. Ball-boys’ job broken down by TV earlier (9)
RETRIEVAL
(TV EARLIER)* (*broken down)
14. During set, gently wobble tooth (8)
SPROCKET
during SET, (P (gently) + ROCK (wobble))
16. Watch most of the top players (3)
SEE
SEE[d] (top player, most of)
19. Faulty serve abandoned, foot out (3)
LET
LE[f]T (abandoned, F (foot) out)
20. Sportswriter ate strawberries covering replays (8)
ITERATES
[sportswr]ITER ATE S[trawberries] (covering)
23. Winner back from warm-down in time for celebratory drink (9)
CHAMPAGNE
CHAMP (winner) + ([warm-dow]N (back) in AGE (time))
24. Enjoy grass with new distinctive feature (5)
TOKEN
TOKE (enjoy grass) with N (new)
26. Five down, D Evans is jittery in chair (5)
SEDAN
(D E[v]ANS (V (five) down))* (*jittery))
27. In front, party prepared to queue overnight? (3,2,4)
SET UP CAMP
Double definition
The "set up" camp being the party in front (in tennis) – I'm not totally sure about this to be honest
28. Some left confused by H Watson’s oppo (6)
HOLMES
(SOME + L (left))* (*confused) by H
29. Got in touch with Queen’s Club (7)
RANGERS
RANG (got in touch with) + ERS (Queens)
DOWN
1. British rugby coach turned up for Wimbledon, say (6)
SUBURB
(B (British) + RU (rugby, union) + BUS (coach))< (<turned up)
2. Rybakina’s back to win once more (5)
AGAIN
[rybakin]A (back) + GAIN (to win)
3. Ball hit over the baseline probably not a winner (4,4)
LONG SHOT
4. Sport that might make Swiatek grin (11)
WATERSKIING
(SWIATEK GRIN)* (*might make)
5. Skirt that would give offence in church (3)
RIM
[c]RIM[e] (offence, if it were in CE (church))
6. Unhappy about wait, finally Iga’s outside show courts (6)
STADIA
(SAD (unhappy) about [wai]T (finally)) + I[g]A (outside)
7. Upstart pair train regularly against tennis’s third and first pairs (9)
ARRIVISTE
[p]A[i]R [t]R[a]I[n] (regularly) + V (against) + [tenn]IS + TE[nnis] (the third and first pair of letters in "tennis")
10. A Murray we heard at the GP’s, for a Zimmer frame? (6)
WALKER
Double (cryptic) definition
Murray Walker being a commentator at the GP (Grand Prix)
13. Youngster H Rune full of energy, literally (2,3,6)
TO THE LETTER
TOT (youngster) + ((H + LETTER (rune)) full of E (energy))
15. To finish, professional breaks chubby little Italian (3,4,2)
PUT PAID TO
PAID (professional) breaks PUTTO (chubby little Italian)
17. Recognises Prince’s logo on strings (6)
PLACES
P (Prince's logo) on LACES (strings)
18. Start to plan out a win on the Fritz serve (4,4)
WAIT UPON
(P[lan] (start to) + OUT A WIN)* (*on the Fritz)
21. Small mount within Elysium’s borders — Henman Hill’s one (6)
EPONYM
PONY (small mount) within E[lysiu]M (borders)
22. Wesley goes for a 3 (6)
SNIPES
Double definition
Actor Wesley Snipes, and 3 down being LONG SHOT
25. J Sinner (5)
KNAVE
Double definition
J meaning Jack in a deck of cards
27. Relative runs away from men like Andy Murray (3)
SIS
SI[r]S (men like Andy Murray, R (runs) away))
Thanks, Guy and Teacow!
Nice to be back. Thanks to all.
SET UP CAMP
I have a different (but tentative) parsing:
party=CAMP (as in a political camp/party)
prepared=SET UP
In front=In front of? CAMP in front of SET UP
Def: to queue overnight
Great puzzle, lots of fun. And I don’t even like tennis that much (I had to look up who J Sinner was to fully appreciate the cleverness of 25d). Thanks, Guy and Teacow.
27a: SET UP = in front (as in being a set up in a game of tennis), CAMP = party. The definition is “prepared to queue overnight”. I heard on the radio this morning that something like 5,000 people did exactly this last night in order to get in this morning. Bonkers!
Oh yes, forgot to say – 10d is genius. Bravo, Guy!
As expected from Guy, this one needed a bit of thought. I liked the way in which the tennis theme in the clues didn’t “spill over” into most of the answers, as I had expected it to when I started. I wasn’t sure about SET UP CAMP but I think Widdersbel @2 has worked it out correctly.
I liked the Murray WALKER reminder, the def for SNIPES and the oblique ‘nerves’ def for BRAINSTEM.
Thanks to Guy and Teacow
Widdersbel@2
SET UP CAMP
Your parsing is quite convincing. Thanks.
Actually, Teacow has parsed it on the same lines (I read it properly only now).
It’s been lonely without 15². I’m delighted to see a puzzle that plays to my strengths and I’ll be glued to the Wimbledon fortnight. I particularly liked SEDAN for ‘ five down’, SNIPES for the surprising reference to Wesley, HOLMES, the neat SWALLOW and the clever use of Swiatek in 4d.
I guessed KNAVE but needed Teacow’s blog to see the use of ‘j’ (of course!)
Very entertaining. Thanks to Guy and Teacow.
Great, fun crossword.
Some very clever tennis-based misdirections
I guess 16a should be SEE(ds).
Lots of fun with this. Thanks to Guy and Teacow.
I read 22d as Wesley=Snipes and goes for a long shot as “snipes” as a sniper does?
Is that is meant in double definition.view of the clue?
18d On the Fritz, really? Whatever next.