The 2003 Siebel Open was a men's international tennis tournament. Lleyton Hewitt was the defending champion but did not compete that year.

Andre Agassi won in the final 6–3, 6–1 against Davide Sanguinetti.

A champion seed is indicated in bold text while text in italics indicates the round in which that seed was eliminated.

	First round							Second round							Quarterfinals							Semifinals							Final
	1	A Agassi	6	6
	WC	M Chang	4	2				1	A Agassi	6	6
	WC	M Philippoussis	3	6	6			WC	M Philippoussis	4	1
		J-R Lisnard	6	3	3										1	A Agassi	63	78	6
		J Vacek	4	2												V Voltchkov	77	66	1
		V Voltchkov	6	6					V Voltchkov	w/o
		B Vahaly	4	3				5	T Martin
	5	T Martin	6	6																		1	A Agassi	6	6
	3	J Blake	6	6																		3	J Blake	4	2
	WC	W Black	2	4				3	J Blake	6	6
		T Dent	2	65					H-T Lee	3	2
		H-T Lee	6	77											3	J Blake	7	6
		K Pless	6	1	4										7	N Davydenko	5	2
		R Ginepri	4	6	6				R Ginepri	4	6	3
		J Morrison	3	6	61			7	N Davydenko	6	3	6
	7	N Davydenko	6	2	77																								1	Andre Agassi	6	6
	6	D Sanguinetti	6	67	6																								6	Davide Sanguinetti	3	1
		Ž Krajan	2	79	1			6	D Sanguinetti	77	7
		M Fish	6	4	77				M Fish	61	5
	Q	D Tursunov	2	6	63										6	D Sanguinetti	6	7
		V Spadea	6	6												V Spadea	1	5
		L Burgsmüller	3	4					V Spadea	6	6
	Q	T Reid	2	0				4	J-M Gambill	3	3
	4	J-M Gambill	6	6																		6	D Sanguinetti	6	62	6
	8	K Carlsen	7	3	77																		J Gimelstob	4	77	3
		M Norman	5	6	63			8	K Carlsen	6	3	7
	Q	P Goldstein	6	2	6			Q	P Goldstein	2	6	5
		J Balcells	2	6	3										8	K Carlsen	3	4
		M Llodra	77	1	1											J Gimelstob	6	6
	Q	Z Fleishman	64	6	6			Q	Z Fleishman	1	4
		J Gimelstob	7	6					J Gimelstob	6	6
	2	P Srichaphan	5	2

• 2003 Siebel Open Draw
• 2003 Siebel Open Qualifying Draw